Use your front brake _a lot_. Just not to the point of locking the tire. It'll be 'hello trail' if you do. When you're on the brakes, all the weight is coming off the rear and it makes it easy to lock the rear tire and skid. Locking the rear is no big deal, it just tears up the trail for the next guy and it doesn't slow you down much.
As far as the log at the top goes, Pop a wheelie and get your front over it. When your chainring touches it, you can drive forward on the ring, then pop the rear over it. If this is the technique that many people have used, the top of the log will be seriously chewed up by all the chainrings. You only have to be careful to not slam the chainring into the log too hard. They can take a pretty good shot, but doing this wrong will bend stuff up.