Definitely don't choke up on the brakes. I have had to teach myself these things the hard way. Feather the brakes, and look for spots you can use them--coast through the rest.
And when they say stand up and lean back, remember to lean WAY back. A friend who is a retired pro mtn bike AND dirt bike racer helped me out quite a bit. She had me so far back over the bike that if I had sat down, I'd have been on the fender not the seat.
Also, don't tense up your arms/shoulders. Let the front end do what it needs to--if you fight it, you will wear yourself out AND crash.
Good luck!
Oh yeah. If the engine is stalling, as long as you can get yourself pointed downhill, you can just start coasting and compression start the bike again. Just get rolling a little with the clutch in, put the bike in second, and let the clutch out at the same time as you bounce a little on the pegs (that's so the rear tire doesn't just slide). No need to wear yourself out kicking. Once you're going downhill, the engine will keep itself running as long as you don't lock up the brakes too much. Just click it back down into first, and stay there.
If you're riding trails a lot, I'm fairly sure someone (probably Steahly) makes a flywheel weight for the KX100. That will keep it from stalling as easily, and also make the power feel smoother. If you ride trails a lot, they're pretty nice to have.