Bunnyhop

Just kidding.
Helmet, find the one that fit you most. Helmet would protect you about the same cheap or expensive, the expensive ones offers better adjustment and vents as well as colors. If you don't already have one I suggest full finger gloves and sunglasses.
Next would be Hydration pack, spare tubes, patch kits, mini pump, and multi tool, you are ready for almost anything on the trail.
As for practice the best and most fundamental is balance.
Practice slow speed turning, turn your bike in a small circle, smallest you can do at the slowest speed you can. Track stand is another important one. When practice new skills it's ok to put the flat pedal on and lower the seat.
Check out this link from Bikeskills lots of good instructions. Have fun on the trail.:thumbsup:
Edit: forgot the link
http://www.youtube.com/user/bikeskills?feature=chclk