Core work helps a lot, as well as exercises that focus on balance and dexterity. I like using a Swiss ball in combination with a theraband
Wall sits, leg lifts, planks, one leg squats, regular squats, and lots of Cardio! That's what I do and it really seems to be keeping me fit for riding :)
Cardio is very important for riding. You can do this by running, biking or jumping rope for 45-60 min. Squats and lunges are very good for strength.