"focus on curling up, rather than sitting up with your back straight" to clarify - don't do a straight-backed sit-up, but curl instead.
also - the thing about not locking your hands behind your head is to avoid straining your neck - you need to make sure it's your tummy doing the work not your neck.
when you're doign the curl, really focus on t he muscles you're trying to work.
oh, and finally, once you're done, avoid eating half a ton of choc brownies. that's my problem. abs like rock, but covered with a lumpy layer of muffin tum