Take a look at "The Mountain Biker's Training Bible" by Joe Friel. When I was still racing [before I realized I had no talent], I used it to plan my workouts for the winter. The leg workouts helped tremendously, and I got a lot stronger. One thing I did that made a big difference was to do "single-leg" exercises. For example, do the leg press one leg at a time with half the usual weight. This will help you to figure out if you have a strength imbalance between your right and left legs [most people do]. You can also join a gym and ask them what to do. Most gyms have decent staff, but watch out for Juice Monkeys.