Leg lifts

Lifting the legs naturally activates the core muscles.  Kickboxing also uses all the core muscles to lift the legs for different kicks. Tae Bo Advanced DVDs are all essentially a series of ab workouts with a cardio pace, incorporating advanced leg lifts and different kicks to strengthen the core.

Side bends

Lightly work the abs by bending in different ways.  Add weights that challenge you. Again you will see many of these types of activities in the P90X series, especially the Core Synergistic video. Various types of yoga utilize side bends without weights, but don't expect them to be easy.

V-sits

Legend has it that Bruce Lee could hold a V-sit for more than 30 minutes. To perform, balance on your butt and extend your legs and arms out so that your body creates a V. Give this Pilates move a try and you’ll feel how difficult it is. The V-sit activates the entire body and also requires a mental engagement that is truly exhausting.  Variations on V-sits are leg climbs and leg extensions of varying difficulty.

Last word of advice

Perhaps all the crunches and ab work you’re doing is not showing off your abdominal muscles because of one thing that has nothing to do with exercise at all: fat. If you have a layer of fat on your abs you can do planks, push-ups, leg lifts, side bends, and 30-minute V-sits, and you will get strong—but you’ll never see a muscle. Follow a weight-loss plan if this is your concern. Not interested in being in show-shape? Don't despair, strengthening your abs will make you feel better, give you better posture, protect your back from wear and tear, and can actually lengthen your life.