The Health and Fitness Blog has a great animated gif of a core fitness routine using a swiss ball. I have worked this into my workout regimens at times and it is a nice break from just doing situps and crunches.
You lay with your head and shoulders on the ground and your arms spread out from your body to stabilize you. Your ankles should rest on a swiss ball with your legs straight. Curl the ball towards you by bending your knees (the swiss ball will wind up beneath the soles of your feet). Finish the move by lifting your abdominals outward to create a straight line from shoulders to knees. Reverse the motion until you are laying with your ankles on the swiss ball again and your legs are straight. That is one repeat.
I do not usually count my reps with an exercise such as this; instead, I go for one or two minutes per set instead of a set number of reps. This will work your abs and your stabilizing muscles, which will help improve your balance. Don’t worry if it takes a little practice on the swiss ball to get used to the movement.