Hi, Mary!
I work abs 5 times per week; two of these workouts are in the pool during the aqua aerobics classes I teach. I use hand buoys to keep the head above water, and do series of V-sit-type crunches with the legs in various positions that really bang the rectus abdominus and obliques; plus, doing prone flutter kicking recruits the abs for stability.
For my land workouts, my fave ab routines these days are Cathe's CTX-Kickbox ab routine and her CTX Power Circuit ab routine, both of which include planks and other core stability work as well. I like to put a pair of 1-lb hand "gloves" on to increase the crunch work. I also like at times to place my heels on a low step (4") rather than on the floor to vary the recruitment of the hip flexors.
I plan someday on bringing stability ball work into the mix, and I'm drooling in anticipation of the new Cathe Ab Hits DVD that's available for pre-order.
I think your best bet is a good combo of supine crunches, V-sits, bicycles and reverse crunches combined with planks!
Annette