Most of these recommendations are just common sense, but they're still worth mentioning...

Ride on open trails only

Respect trail and road closures (ask if not sure), avoid trespassing on private land.

Reduce your impact

Be sensitive to the dirt you're riding on. If it's been raining cats and dogs for a couple days, it's probably not a bad idea to wait a bit before hitting the trail.  Riding rutted-out trails isn't fun, and the folks who decide which trails stay open don't like to see it happen.  Avoid skidding your turns - it's really hard on the trails.  Don't leave trash in the woods.

Control your bike

If you're riding out of control, you're probably going to crash.  Trees and rocks may look soft, but they are surprisingly firm.  Don't be the rider who has to be heli'd out of the woods with a broken leg.  It's less fun than it looks, and you can't ride when you're in the hospital.

Always yield the trail

If you see a biker or horse or a hiker on the trail, slow down and say hello.  If you're in a group, it's a good idea to let other trail users know how many of you there are.  "Hi, there's 4 more behind me!" goes a long way to improving relations between trail users.  Oh yeah, don't spook horses.  Getting kicked by a horse is like being beaten with a 4x4 post.  It sucks. 

Never spook critters

All animals (including dogs) are startled by an unannounced approach, a sudden movement or a loud noise. This can be dangerous for you, others and for the animals. Give animals extra room and time to adjust to you. In passing, use special care and follow the directions of horseback riders.

Plan Ahead

Know your bike, your ability and the area in which you are riding - be prepared.  Carrying food and water is always a good idea.  First aid kits are a good idea too.  Weather in the mountains can change really fast, and a light jacket jammed in the bottom of your pack can be a real life-saver.

Have a good time!

Enjoy the ride!

* Modified from IBMA's Rules of the Trail to address local riding conditions and laws.