I've read many a time that sit-ups aren't the best way to do it, I think it's cos they only really work the muscles one way or something! Things like planks and side planks are fab though!
I've actually just remembered, when I had my small one, my sister told me to sit upright in a chair and pull your muscles in and count down from 100! It actually worked quite well from what I remember! I'll have to start doing it again