Lay down...then sit up?
yuk yuk yuk.
Typically, I'd lay on the floor, knees bent, hands clasped behind head (but not used to pull head up off the floor) and lift enough to feel the crunch but not touch the knees. Hold for a split second and lower down slowly.
You can really feel the burn quickly, that way.
An alternate way is, as you're raising, twist and touch your right elbow to your left knee and then go the other way (works on the obliques.)