Do you have access to an indoor trainer?
www.lwcoaching.com Lynda Wallenfels is a coach/trainer and endurance athelete who designs programs to be used on a trainer, solo. She's got base training programs, solo racer programs, endurance programs etc.

Quote Originally Posted by limewave View Post
Strength training I have found valuable for increasing power and stability on the mountain bike:

Plank
Side Plank
Plank--curl knee into chest, hold, then stretch out, repeat. Alternating legs.
Side plank with hand weight: With free hand, lift weight straight into the air then slowly curl down under your side and back into the air.
Plank to Dog using a exercise ball

Scissor Jumps
Box Jumps
Isometric Squats
Single Leg Squat Balance Press


I don't do all of these in one session. I rotate throughout the week and throw in some upper arm workouts, sit ups/crunches, and general leg-toning for a well rounded strength workout.

Another good workout is Stair Running
most of those are more core and plyometrics than true strength training. All still good mtb, though! Lynda' programs give you core to do on the days you aren't on the trainer.