A lot of rock climbers use a stress ball to strengthen their fingers. Also doing basic weight lifting can help, but I would say the most important part is having strong fingers.
Any exercises that strength your upper body and legs are good. Train on the rowing machine for upper body. Do weight training to strengthen your legs. Repetition of these combined exercises is important.
One exercise learned from martial arts is to do push ups using your fingers instead of your whole hand. This strengthens your fingers as well as your upper body.