I've found it very helpful to concentrate on looking at my bellybutton while doing the exercises that (used to) strain my neck. Try focusing on your bellybutton (NOT the ceiling!) for the hundred, roll-up, rolling-like-a-ball, open-leg rocker, neck-pull, teaser, and seal. Since I haven't taken classes in a while, I'm not doing the more advanced exercises.

Looking at your bellybutton forces you to round your neck and upper back, whereas looking at the ceiling results in a cantilever effect with your head, which is very heavy!