Use a physioball for any abdominal exercise. I found that most of my back pain during crunches was caused by a lack of back support (solved by the physioball) and a lack of strength in my core (solved by working on my core on a daily basis).
I have spinal lordosis, which means my spine curves inward towards my belly button too much, and a forward head, which means my ears do not line up with my shoulders as they should, and it looks like I stick my chin and butt out in an exaggerated manner. These are ongoing posture concerns I have been addressing by doing work on the physioball, and I've found that strengthening my core has definitely allowed my posture to straighten up.
Taking an actual abs class and learning the correct movements for abdominal exercises is also a good idea. My core workout consists of exercises from an abs class, some yoga, and this Pilates move called the Hundred:
http://pilates.about.com/od/pilatesm...heHundered.htm The yoga serves to stretch out my abs and back between sets of traditional crunches, bicycle crunches, reverse crunches, bicycling, and the Hundred. I usually do a full Sun Salutation as part of my core workout, working Cobra into the sequence:
http://www.yogajournal.com/poses/471_1.cfm I also do the cat-cow sequence (all fours, start with a neutral spine, then push your belly button down towards the ground, exaggerating the curve of your lower back. Hold for 30 seconds. Then push your belly button towards the ceiling, arching your back like a cat. Hold for 30 seconds. Return to neutral spine position) to stretch out my back and work my abs at the same time.
But starting with a physioball is probably your best bet.