Use your gym's roman chair. This piece of equipment allows you to alter the angle at which you work your hamstrings. Make sure you keep your feet straight ahead and your spine neutral, to avoid injury from incorrect form. Swimming is a particularly good cross-training sport for the upper body. I do a lot of mountain biking, and in mountain biking, many bikers don't realize how much upper body strength you need because shoulders and triceps easily fatigue.
Strength training is important, too.
In mountain biking, you're changing your positions at the bars with hand positions and shoulder positions. You have to oppose the movements of the terrain by having a strong upper body. Another good shoulder strengthener for cyclists is the hanging clean and jerk. Stand holding a barbell above your knees, and in one movement, move the bar from your knees to your chest. Jennings urges cyclists, who are often lean, to do this with a lighter weight.