Remember try to do all your braking before you start to corner. If for some reason you misjudge it (and you will, I even do it on my local track occasionally) don't freak out, just lean back a little bit more than usual, leave the front brake alone and drag the rear brake through the corner but don't lock it up. As yo uare in the corner lean and pull back on the handlebars a bit and throw the bike around the corner. However try to keep yourself from gettign into this situation it can get really sketchy.
Braking in general: front brake will give you more power, use it when on a straight line coming into an obstacle.
If there is an obstacle infront of you, rock, rock garden, drop, stairs......... do not hold the bars too tightly. Let your arms be able to move freely and let your legs move freely. The idea is to allow the bike to move underneath you but keeping your torso and head still. Watch all the best DHers, their heads do not jolt or move, it is steady as if they were on rails. Also try and keep your head upright, even when leaning into a corner. Finally, you go where you look! If there is a big rock onm the track, don't look at it and go, I better keep away from it. I can tell you now, you will most likely hit it, ignore what you don't want to hit.