For the crunches, make sure you are situated on the ball correctly and stretching back on the ball and the only using your abs to crunch up slowly. Make sure you aren't using your upper body and head for any momentum or that you aren't using the ball itself for any momentum (ie letting it roll). If you put your hands over your head and keep your arms glued to your head and don't use them at all to pull you up, this will also be challenging. I promise you that crunches done slowly, deliberately, and correctly are a great core exercise.
