Hopefully Novanutcase will correct if this is a wrong way of strengthening abs, but it worked for me and I found it much easier than situps...
I used my (ex) boyfriends weights, a couple of round ones on a bar, so the bar can roll inside the middle holes. Then stand on your hands and knees, lean on the bar with the weights on with your hands and slowly let it roll forward, keeping your knees/legs in the same position on the floor. You will feel that to not end up falling flat on your face you'll need to use your stomach muscles to hold your body straight, then pull back again slowly.