There's definately some good advice given everyone else... I have to agree with a lot of the comments made!! Putting the seat down just in general, particularly when you are riding single track makes a big difference. Not only does it lower your centre of gravity, but it also allows you to move alot more freely around your bike. Give it a go, and you will immediately notice the difference (at least I did!!).

In terms of your descents - lower your seat, and go for a ride around the park on the flat. Ride at a fairly steady pace (not too fast) and see how far forward, and how far backwards you can get (obviously without having a crash!!). If you're having trouble with your descents, it may be because you're just not putting your weight far enough back (hence feeling as though you're going to endo!). Once you put the seat down you should be able to put your whole body above the rear wheel... Give it a go in the park and get used to how far back you can go... Then give it a go on the trail.

Hope this helps!