I went on my first snow ride this winter. Here's a few things I've learned:
1. Pick a trail that is somewhat packed down or doesn't have a ton of snow. It won't be much fun to break trail in 8 inches of snow. A few inches of light powder, or better yet, more used trails or railtrails used by snowmobiles will be a lot more fun and you won't get discouraged as fast.
2. Whear warm clothes. Nothings worse than getting out on the trail for 45 minutes and suddenly realize that your starting to get real cold. Wear layered clothes, warm gloves, and some type of ear warmers/ some kind of skull cap-thing under your helmet. Specially designed bike tights and winter riding gloves and ventilated jackets will work best. If you've got a book store nearby, pick up an issue of MBUK. A recent issue had reviews of winter riding gloves for mountain biking. Glasses or goggles will help protect your eyes.
3. Run the tires on lower pressure. You'll want the traction to keep from sliding around. The most important thing to remember is that the front tire is the most important to keep under control. It it slides out, you go down. If the rear goes out, you can control it. I run a home-made sutdded front tire on my bike in the winter time when the trails are snow or ice covered. Quick steering movements are bad, and you'll have to be prepared to keep your speeds down in the turns or rough sections no matter what your running for tires.
4. Run platform pedals. Nothing worse than going down still clipped into the SPD's. And you will go down faster on the ice and snow, and no-matter how confident you are, you'll find yourself still locked in because of the speed of the fall. Also, wear a warm shoe/sock combo.
5. Watch your brakes. You don't want to be locking them up, as it will certainly send you out of control in most cases. If you've got discs, its nice, but if you've got V's, imagine a rainy ride times about ten. Frozen snow and slush on the rims can render the brakes nearly useless. Another reason to keep the speeds down somewhat. It might be worth a quick drag on the brakes now and then to keep slush from building up and then freezing on the rims.
And, dry the bike and all of the components off afterwards. Relube and clean sections that got a lot of exposure to the snow and slush. This will keep off rust and make sure the bike is ready for the next ride. The perfect setup is to put the bike in a warm, dry place while you do this. I have a woodstove in the basement, so I have an ideal place to put the bike after drying and relubing to ensure that everything drys thoroughly.
It's late so I've probably forgotten some stuff, but this is all I can remember for now. I'll post more later if I remember more. Good luck on the new experience!
-Moab