This is one of my favorites both for my PT patients and for my coaching clients. There are some ways to make the exercise easier so you can do more reps. For one thing, using a larger ball actually makes it a bit easier, to a point. I usually start my clients with a series that works out to 30 reps and we do one to 3 sets.

The version in Zen's link is the hardest using 2 legs. The easiest version I do has the athlete in the same starting position, lift the hips about 2 inches off the floor and just roll the ball in and out. The next easiest is to roll the ball into the bridge position (hips up, knees bent), keep the ball stationary and just lift the hips up and down.

My favorite set is to start with 10 reps of the hardest version of the exercise, then immediately do the next easiest one (hip lifts with a stationary ball) for 10 reps, then finish off with the mini hip lift and rolling the ball in and out. It's a really nice way to manage 30 reps with frequent changes so you're less likely to cramp up during the exercise.