It's always fun to ride ATVs in the snow, but it's not fun getting stuck. Getting unstuck can be as easy as getting stuck too.

[edit] Steps

Assess how deep the snow is. If the tires are spinning then try to dig out the under carrige so the weight will be put on the wheels, you can usually rock side to side to get out.
Accelerate away from deeper snow. Depending on how you got stuck, accelerating backward may be necessary remember not to accellerate too fast.this can cause the tires to just dig into the snow making the situation worse.
Move your body to the oppisite end that you are going. the direction you're accelerating. This can help you get out especially if in 4x4; also, you shouldn't be gunning the throttle.
accellerate slowly as you start moving. best way to get unstuck is to use a winch or have someone else pull you out with an atv while accellerateing tword them.