Things to learn - PLAY with your bike at slow speeds to help you learn to deal with anything in the forest and races.

coping with steep downhill - body positions
coping with riding uphill - body positions
cadence ('revs') - revolution of pedalling, what is optimum? [80-100]
jumps - downhill and on the flat (bumps)
balance - hopping
falling - with your bike (eg if couldn't escape toe clips or cleats)
falling - without your bike
experiment with your brakes at different speeds - front only, rear only, and combinations of both (but at different 'pulls'), 'feathering'
braking - think ahead, anticipate
sharp turns using brakes - fast (rear slide) - slow: front wheel pivot or rear wheel pivot?
cornering: outside pedal at 6 O'clock, cover brakes, leaning - fast (banking) or slow (use both brakes)
gear changing: anticipate - think ahead and change early with speed (and revs) up
basic log pulls (or curbs) = front wheel lift, clear and ride over
advanced log pull (bunny hopping) = flat jump, both wheels over obstacle => front wheel lift and rear wheel (pull up with feet => toe clips or cleats)
curb hopping - side on (eg if faced with hill straight away)
drop offs - slow (body over back and roll) Vs faster (wheelie or jump)
carrying your bike over obstacles or up steep hill - wheeling Vs carry over shoulder?
dismounting at speeds so you can run
different terrains - racks, mud, sand, water (shallow and deeper). Have a play; what works?
wheelies!

How to learn these?

Practice on flat eg. on grass; set up own slaloms of tight turns and obstacles.