Sexy Summer Abs Workout Planks, side plank lifts, knee hugs and a whole lot more to get those abs on fire! In between reps on the knee hugs try not to let your feet touch the ground; this will make it even harder and work your lower back as well
JUST DO IT!: custom printable workout exampley I like that there are stability ball exercises too (see the last 2 exercises on this example workout)! Workout Labs give full instructions on how to properly do the exercises.