Working with someone really is the best approach.

Here is a basic ariticle on offroad and on-road. A lot of the same stuff applies.
http://www.rei.com/online/store/Lear...cldescendf.jsp

When MP and I were descending parts of Mines Rd, she commented that I descend like a mountainbiker. I've consciously ingrained those habits because of trail obstacles and I'm incredibly more relaxed on singletrack. However, after her comments, I'm trying to keep my butt in closer contact with the saddle and, consequently, my speed is going up as I'm able to extend my outside leg/toe more and push harder. Of course, if I crash on the trail, I'm going to blame her.