what are crunches and how you do them?
One of the problems with putting your hands on your head is that it encourages you to use your arms to jerk yourself up instead of using your abs entirely.
Better to cross your arms on your chest or to make fists and hold your hands next to your ears without holding on.
I like to do crunches slowly. I take 3 seconds to slowly roll my shoulders off the floor, hold them up for 3 seconds, then take another 3 seconds to slowly lower my shoulders back to the floor. To make it even more demanding you can contract your abs a few times while you are holding yourself up. Another variation is to roll one shoulder up and twist toward your opposite knee.
An even better ab exercise is one called the bicycle maneuver. It is hard to describe but there are a few videos of this exercise on the web to demonstrate.
Its the most common abdominal excercise to get six pack abs. You do them by lying flat on your back on the floor, knees bent and your feet on the floor. You can hold your hands basically anywhere you want, most common is around your head. Then roll your upper torso forward. :)
situps but you just go half ways