It's tough and awkward to do all of the elements of your workouts with the standard weights that are meant to be put on the end of a bar. I find this especially tough for enhancing an ab workout.
Smart Bells ($89.95) are sculptural weights designed to conform to the shape of the body. Also, there are two handles which make gripping and moving the weight much more secure and comfortable. A center indentation cups the head and the curve mirrors the shape of the body for comfort during crunches, lunges, bench presses or whatever it is you do that you like to add a little weight on.
If you're really into Smart Bells, there is an entire routine (complete with video) that you can get as well. Check it out and order your Smart Bells from Thinkfit.com.