This is one thing that bugs me when I do ab crunches. Most of the time I gave up because there's too much stress on the neck. That's also one of the reason I don't enjoy Pilates class.
Last month I attended Min's Pitales class at Chayo Studio. Her class was very enjoyable, it was like a mixture of fast-pace Yoga and slow-pace aerobics. With the music playing at the background her movement was like a graceful dance. And the most important thing she had taught me - the head/neck position during ab crunches.
Previously I always did it with my chin tucked in.
Now I tried her method, keep my head and upper back neutral and neck relaxed.
I now look at the ceiling instead of my belly during ab crunches, and avoid pulling my upper body up using my hands. It helps a lot.