This is a common complaint. In fact, in my many years as a personal trainer I have heard this dozens of times. There are a few things you can do to make your neck more comfortable while performing crunches. I know this will sound strange, but the first thing you may want to try is placing your tongue on the roof of your mouth while crunching. This is the anatomical resting position of the tongue, and it will help to recruit the muscles near the surface of the neck that best support your head. When your tongue is in any other position the much weaker muscles near the cervical vertabrae must support the head. These muscles are easily strained and/or injured when overtaxed. Another thing you might try is resting your head on the floor briefly between reps. This will lessen the stress on your neck by allowing these muscles to relax momentarily. Just don't rest too long or you will compromise the training effect on your abs. Finally, do not interlock your fingers and place them behind your head during crunching movements. This is probably the number one reason for neck strain during ab work, for as you begin to fatigue you will most likely begin to pull on your neck and head in order to do more reps. This can overstretch connective tissue and injure the delicate neck muscles.