STRATEGY 1 :  Pretreat (cold stratify) seeds and sow  for 'forced' germination times in the Spring.

Pre-treating seeds (cold stratification) is a simple measure you can take which will break a seed's dormancy causing the seed to be more ready to germinate. By subjecting the seeds to this pre-treatment you are really only providing them with the effect that mother nature would have had on the seeds had they been left to their natural course. However by applying the pre-treatment yourself in a controlled environment such as your refrigerator, you are speeding the process up and are better able to control and diminish factors detrimental to a seed's survival had it been left to make it on its own in the wild. By cold stratifying the seeds you are able to affect the time frame under which the seeds will germinate. By not cold stratifying the seeds (Strategy 2.) you will have to be content to accept nature's timeframe.

The pre-treatment or "stratification" of seeds is not a fixed science and one shouldn't be overly concerned about exact lengths of pre-treatment time. If it is recommended that a particular species of seed will benefit from 2-3 months cold stratification, this only means that past experience finds that this seed's "dormancy" is usually overcome by approximately this length of cold stratification and as such the seeds are more susceptible to germination and will generally sprout at a more "even" rate. 

Within reason, a  rule of thumb might be said to be "the longer the pre-treatment... the more uniformity in germination rates after sowing" - meaning the majority of seeds will sprout closer together in time rather than spread out over time.

What follows is not rocket science but a few simple basics in sound horticultural practice where seed germination is concerned and is applicable in many respects to any type of seed germination, especially in respect to the benefits of sanitary practices.

A note on "Cold Stratification" and  "Seed Dormancy"

Many tree seeds have what is called an "embryonic dormancy" and generally speaking will not sprout until this embryo dormancy is broken or overcome.
In the wild "seed dormancy" is overcome by the seed spending time in the ground through a winter period and having its hard seed coat soften up a bit by being subjected to moisture and bacteria. 
By spending time in the ground the seed is undergoing a natural form of "cold stratification" or pre-treatment.

This cold moist period triggers the seed's embryo, moisture is absorbed causing the seed and its embryo to swell and its growth and subsequent expansion eventually break through the softened seed coat in its search for warmth, sun and nutrients.

In its most basic form, when we control the cold stratification process, the pretreatment amounts to not much more than subjecting the seeds to storage in a cold (not freezing) and moist environment for a period found to be sufficient for the species in question. 
This period of time is often and usually found to be somewhere between 1 through 4 months.

To accomplish this mix your tree seeds in a clean plastic sealed or ziplock bag with thoroughly moistened vermiculite (or peat) and place in the bottom vegetable/fruit compartment of your refrigerator (not freezer). 
Use ten or twenty times the amount of vermiculite (or peat) as tree seeds. 
It is important to thoroughly but only slightly dampen the vermiculite (or peat). 
Excessive moisture can cause your seeds to mildew and grow mouldy in the bag. 
As such, err on the side of drier rather than wetter. 
To give you an idea: you should not be able to squeeze any dripping water out of a handful of peat or vermiculite after thoroughly and uniformly moistening it. 
(moisten the peat completely then squeeze it of excess.)

*After undergoing the recommended period of cold stratification in your refrigerator the seeds are ready to be removed and sown in a warm situation in flats, the nursery bed or pot for germination. Preferably one should try to time this event to occur with Spring.

Tips On Sterile and Sanitary Measures:

Many sources recommend using peat when cold stratifying seeds in the belief that peat is highly sterile and pathogen free. This is fine as long as you make sure to not use peat overly saturated with water-it is less forgiving than vermiculite. Using peat which has not be squeezed of excessive moisture often leads to fungus or mould growing on the seeds and this is especially the case if no fungicide has been applied. This mould, if left unchecked, can cause the seeds so much injury as to prevent germination. While I often use peat as a stratifying medium my preference is to use vermiculite which is sterile, inexpensive and found in just about any garden center. If you do use peat to stratify seeds, acquire a newly bought bag of clean and dry peat moss of the type found in garden centers rather than any old or used peat from the garden and keep a closer watch on your seeds while they are undergoing the pre-treatment and if possible use a fungicide. Usually the number one reason for the outbreak of fungus or mould during the stratification process is excessive moisture in the peat in combination with excess bacteria.

Tips On Preparing Your Cold Stratifying Medium:

You can just use water to moisten the peat or vermiculite, or:
Many sources do not mention that you can also use a little fungicide when moistening your stratifying vermiculite or peat so as to help prevent mould or fungus outbreaks. 
I often use a liquid fungicide like "NO-DAMP" (or any similar horticultural fungicide) which is inexpensive, comes in a little bottle and again, usually found in any garden center. 
(Fungicide Substitute: 1 or 2 parts laundry bleach to 30-50 parts water.)
You can moisten your stratifying medium (peat/vermiculite) by applying a fungicide/water mixture to the 'medium' by using a spray/spritzer bottle according to the water dilution strength indicated on the "NO-DAMP" bottle, however exact mixing ratios are not critical here. 
If you are pre-treating many seeds you should spread your seeds/stratification mixture into a few different baggies rather than putting them all into one baggie. That way if you have a fungus outbreak it will be restricted to only some seeds.
If you forego the use of fungicide and just use water to moisten your vermiculite/peat-keep a closer check on them.

If you do have an outbreak of mould or fungus, all is by no means lost. Simply remove the tree seeds and respray them with your fungicide mix (so as to kill the mould), then place them back in a new baggie with new, slightly moistened vermiculite or peat. Always keep the ziplock baggie sealed otherwise the medium will dry out quicker than you think. The appearance of a little bit of mildew and/or fungus is not a problem and is to be expected sometimes-however if it becomes aggressive and unrestricted in its growth then take the necessary measures.
It is a good idea to check your stratifying seeds on a regular basis for either fungus or germination. If any tree seeds germinate while in the refrigerator simply remove them and plant.

Any seeds that may be indicated as needing a period of  "warm stratification" followed by cold stratification should simply be subjected to the same measures as outlined above but this time the seeds should be stratified in a warm area first, followed by the recommended cold stratification period in your refrigerator thereafter. I use the top of my refrigerator (outside on top) where it is usually about 62-70 degrees fahrenheit for any warm stratification periods. In many instances, warm stratification followed by cold stratification requirements can also be met naturally by simply sowing the seeds in summer (around July/August) in a nicely mulched bed for expected germination the following Spring. (Depending on species and degree of dormancy some seeds may not germinate until the 2nd spring or even third Spring).

Tips: Soaking.

Because a seed naturally needs to begin imbibing moisture before it will sprout it helps to soak them in room temperature water overnight immediately before placing them in cold stratification. This is often sufficient for seeds with soft coats-however in many cases mere soaking in room temperature water is not quite enough for those seeds with really hard seed coats. Some seeds have what is known as a "mechanical dormancy" which refers to the seed's extremely hard (or thick) and often stony and impenetrable seed coat. When faced with seeds such as this it helps to slightly rupture (or 'nick') the seed coat with a knife or file before soaking (or use sandpaper to reduce the thickness of a part of the seed coat). One can also be a little more aggressive in the way one soaks seeds with extremely hard coats by slowly pouring very hot tap water over them a few times and letting them sit and cool in this over night before cold stratification. These measures are in a way, inducements or short cut tips!,-if given sufficient/extended time in cold stratification most seeds with hard coats will eventually begin to soften up and start absorbing moisture. One can also soak them for a few hours in room temperature water just prior to planting if after undergoing the period of cold stratification the seeds have the appearance of not having swelled or split much.

While the actual amount of time it takes to stratify tree seeds with good effect depends on a number of variables such as species, actual stratifying conditions and the degree or depth of dormancy which is unpredictable and often quite variable from seed lot to seed lot: in a great many cases two to three months is often sufficient. This means that if your Spring starts around May/June you should start stratifying your seeds in January/February so that they will be ready to sow a short while after the beginning of warm weather and the earth has warmed up considerably. You should not sow too early as the ground is still cold and does not make for a happy pre-treated seed. Seed germination is enhanced when the pre-treated seed is sown in a warm moist situation.

Tips On Sowing and Seedlings:

Once the seeds are finished their "pre-treatment" and immediately before actually sowing the seeds I give them a spray/wash of fungicide/water mixture. I also give the hole I am planting the seeds into a good spray as well with my fungicide/water mix.
All seedlings, whether grown in pots or beds benefit from good air circulation which wards off fungus growth and promotes sturdy stems. Pot and nursery bed soil for seed germination is not critical as long as the "soil" is light and 'friable', as well as lightly tamped down but not heavily compacted.
Using a potting soil which is described as being sterile ("PRO-MIX" or similar) to sprout your seeds in will aid you against possible problems with "stem rot" and "damping off": (these problems are more inclined to happen due to the presence of excessive bacteria, excessive moisture, cold earth in combination with and especially due to, poor air circulation!). 
Because these problems are much more likely to occur if air circulation is poor, letting your seeds and seedlings germinate and grow outdoors in the wind and sun warmed earth is better. Some partial, screened or light shade is beneficial for seedlings-they are susceptible when young to withering and damage from unremitting strong or direct midday sun. Japanese Maples for instance are particularly sensitive and one strategy is to only let them only get a few hours of early morning sunlight and thereafter to let them sit it out in strong shade. As they get a little older their sensitivity to direct sunlight is less.

Most tree seeds need only be planted 1/4 to a 1/2 inch deep in order to germinate (exact depth is not exceptionally critical). If you plan on planting your seeds outdoors in a nursery bed, then plant them a little deeper to about 1/2 to 3/4 inch because the disturbance caused by heavy rainfall has a tendency to turn the seeds up. Keep a check on the seeds and push them back in if they do come up. 
Lightly mulching your seed bed helps to provide protection against heavy rain turning the seeds up in the winter but you should lighten or remove completely all the mulch cover in the spring-once any seedlings break through the soil's surface remove all mulch entirely if any has been left on. Make sure to keep the nursery bed uniformly and lightly damp but never soaking wet and never let it dry out completely.

The Fall planting of seeds directly into a nursery bed for germination the following Spring naturally satisfies a seed's requirement for cold stratification. This means you can forego all the above outlined pre-treatment in your refrigerator as the over-wintering of the seeds in the earth accomplishes the same thing. Usually the results are as good (and sometimes better) as those resulting from seeds which have undergone artificial cold stratification and it is in itself a widely practiced means of germination by professionals-not to mention mother nature. (Depending on the species and variable depth of dormancy some seeds will sprout in the second and third Spring.)
Successful seed germination relies to a great extent on gaining a 'feel' or understanding of what seeds need in order to sprout. Experience is both the best teacher and 'greener' of thumbs.

When to Sow/Germinate?

I am often asked "When is the best time to germinate seeds?" Of course the best time for seeds to germinate is in Spring as then they have the whole summer to grow on. That being said, I often have seeds that end up germinating right up into late summer with no problem: (you will find that the seeds have a timetable of their own). 
If you live in colder zones, it can help to pile some leaves and larger twigs around seedlings to give them a little winter protection.
If you intend to germinate naturally than sow your seeds in the middle to late Fall-approximately the time when frosts are just starting to occur-there are no exact times for this.

Another query I often get is:, "Can I start my seeds and grow the seedlings indoors?". This can be done provided your light source is sufficient (strong!) and you subject the seedlings to fairly strong air circulation. If the seedlings do not get both of these requirements they will exhibit spindly growth and will have a difficult time when eventually transplanted to the garden.
It is during the seedling stage that the need for strong air circulation is greatest for the prevention of stem rot etc.,-once they have passed the immediate seedling stage strong air circulation is good for enhancing stem/trunk girth.

Fertilization:

Simple rule of thumb! Don't over do it. If the seedlings are growing in a highly organic and rich soil then they really won't need it-otherwise once a month with 10-10-10 or 20-20-20 is fine. You need only fertilize when the soil has warmed up and things are in full swing-this is in fact the only time they can use the fertilizer.

Key Points:

Seed germination is more of an art than a science.
Pre-treated seeds like moist warm situations for germinating-not sopping wet cold situations.
You will find some batches of seeds sprouting before you expected and some later than expected.
Perhaps the single most important point in cold stratification is sanitary conditions.
Be patient when cold stratifying seeds-give them the recommended time and a few days or weeks extra for good measure.
Think sterile! Use clean/new potting soils and/or vermiculite wherever possible.
Don't over water, over dampen anything.
You don't have to cold stratify if you don't want to! Sow them in the Fall/Autumn in a mulched bed or garden nook for germination the following Spring. Its been working this way for thousands of years! (sow a little deeper 1/2-3/4 in.)
Fertilize your seedlings only when the weather warms up-this is the only time they can actually use it anyway.
Don't give up on any seeds that you planted but did not germinate immediately-this is a frequent outcome and they will more often then not come up in the next spring-they usually just need more time to overcome their dormancy.
Plant as many lilac seeds as you can-your nose will be glad you did when you sit in your garden down the road! :)
Take a walk in your city park in the Fall/Autumn and gather tree seeds for free-you will be amazed at the many old and wonderful species your city park probably has! (City Hall usually has a tree species map too, with each tree wearing an identifying numbered tag.)

Every single tree seed carries a different genetic blueprint and because of this every one that germinates and grows on to a tree will be unique in some way. You will often get a number of seedlings that will really be quite diverse in either flower colour, form, speed of growth etc etc: this is where the majority of "named variants" come from. The majority of trees bought from nurseries, while often superior bred forms are usually also all clones which can be quite boring. Trees grow remarkably fast from seedlings.

*When storing seeds for later planting or for stratification at a later date store them in a sealed plastic container so as to keep them dry and place in your refrigerator. 

**Trees are usually quite forgiving when it comes to what kind of soil they will tolerate. However they don't do well in soils that are constantly water logged so steer clear of planting them in really wet areas. Try to amend your soils when possible with organic matter.