I've been riding offroad for several years and have been on alot of rides with people with differing levels of riding skills. What I see most newbes do, my self included is to go too slow. You end up 'fighting' the bike rather then letting it do the work, this holds true for most tight technical terrain. But I don't mean you should go blasting out of control, feet and legs flailing about.
Also, try and keep you feet on the pegs as much as you can. If you take your feet off the pegs you then begin to rely on your arms and hands to to keep your body from sliding forward on the seat, putting more force on the handle bars, and you won't control the handling nearly as well.
Finally, I've gone down many a hill with the clutch pulled in simply because of the problem you mentioned about locking up the rear tire and killing the motor. Just let off the rear brake a tad when you feel it sliding. And don't be afraid to grab a handful of the front brake, I've never gone over the bars because the front tire locked up.
I surely don't want to sound like I know it all when it comes to off-roading cause god knows i've spent my fair share crashing the through the trees, just saw an opportunity to ramble on endlessly.