I'm still pretty new at MTB, but I have to say that a mixture of riding with friends who are better than me, and then by myself where I can just play around with stuff, has allowed me to improve fairly rapidly. Riding with my friends makes me push my self and see that I am actually capable of doing things at a significantly higher speed than I would by myself and get some help on the more technical sections (I just can't stand to be left behind, even though I know they'll be waiting just down the trail. I guess I'm just too competitive ) Just keep at it, you'll be surprised at how rapidly you'll improve.