Don't coast. You should always be either on the gas or on the brakes. If you are going to moto that KDX, some suspension work is definately in order, the stock fork springs are too soft to even sit on the bike, much less handle any jumps.
When you are on the track, don't try to race the other bikes, that will get you hurt if you have no experience. Race the track.
Stand as much as possible, this will give you better control in rough stuff, and will keep you from being spit over the bars in big whoops.
Learn to move around on the bike. You can't just sit in one position, you have to slide forward for turns, back for acceleration, middle of the bike for bumps, etc...
Keep your elbows up when sitting. This will give you much better control of the bike.
Practice, Practice, Practice...watch other riders when they are going aroud the track to see what lines they take, what they use for braking points, etc...did I say practice?
------------------
1992 KDX 250-FMF porting,two-stage power reeds, Fatty pipe, Power Core silencer,titanium rod,Wiseco Ultra-lite, Pro-Action suspension...Oh my Gawd, they killed Kenny!