My personal trainer gets me doing loads of crunches, also "cycling" with my legs while laying on my back and in the crunch position is a favourite of his. If you put your legs straight up then reach for alternate feet while crunching it will work the outsides of the abs and lower abs. Most important of all make sure you are in the correct position when doing the sit ups, ensure your lower spine is pressed into the floor and that your hips are tilted so that you engage your abdominal muscles. If you don't ache after doing a set of say 10 or so then you are doing something wrong. Always remember if you don't feel it burn it isn't doing anything!