Ding

bicycle crunches - lie on your back, hands by your temples, lift one knee towards you whilst reaching towards that knee with the opposite elbow. then change to the other side and repeat. this works the upper, lower and oblique muscles in the stomach, its very effective.

reverse curls - lie on the floor, raise your legs up, knees slightly bent. then tilt your pelvis up, so that your rear lifts off the ground slightly. aim to 'stamp on the ceiling' with your feet. lower then repeat.this works the lower section of your stomach

lie on your back, knees bent. lift up off the floor slightly and reach over to the left with your hand, as if aiming to touch your heel. this should only be a small movement. repeat, then switch to the other side. this works the oblique muscles.

normal crunches and sit ups are good in some ways too for building up your abs, but its much more effective to do a wide range of moves to work the whole of the stomach. have a go at the ones above to mix up your normal routine. and remember, to always pull in your stomach when doing the moves, otherwise you'll be training the muscle to bulge outwards.
best of luck