How do I prevent neck strain when doing crunches or sit ups?
rest your head on the mat between reps
you should also focus on keeping your cervical spine neutral (not bending your head forward or back).
abs flex the spine, so don't focus on going all the way up (which recruits mostly hip flexors at the end of the movement), but rather simply flexing the spine; if you just want to focus on the abs, once you have flexed the spine, don't try to go up further.
try keep your hands by your side. putting your hands behind your head actually increases muscle involvement of the neck, despite the common belief that it helps to "support" it.
Stop doing them. Use the machine crunch or the roman chair. As far as targeting abs I think Front Squats and overhead squats help ab and "core" strength more than most other weight training moves.
I have several suggestions:
1) Do not put your hands behind your head. People sometimes pull up on their head to help with the crunch. The contraction needs to come from the torso.
2) If you are doing a lot of reps than trying lowering the reps and adding resistance or doing them on an incline. Sometimes strain can be a result of too much volume.
3) Take some time off from doing those exercises.
4) Consider doing full body exercises such as squats and deadlifts instead of exercises that isolate the abs. Someone already mentioned overhead squats. The midsection connects the upper body with the lower body. Any heavy movement that transfer energy between the two works the midsection. Think about a squat. It is the torso that is the tranfer point for the power of the legs. Think of a deadlift. The weight is connected to the upper torso by the arms and the energy of both the upper and lower body is tranferred through the torso to lift the weight.
lie down on your back and raise your heels 6" from the ground for 30 seconds to 2 minutes while keeping your knees straight. you can keep your hands folded on your chest.
then, without letting your heels touch the ground and still keeping your knees straight, start kicking your legs up from the height of 6" to the point where your body is bent 90 degrees. do this as many times as you can.
your upper body and neck will stay on the ground motionless, thus relieving you from the pain while working your abs.