I'm big on working out so I know alot of excersizes to work different muscles. For abs I reccomend laying flat on your back like you are doing a sit up but fold you legs and put them to one side. Lift up the side opposite your legs are placed. This works the sides of your abs, which sit ups don't get.
If you have an excersize ball you can do some really good ab workouts too. One of the best is put your legs on the ball and then put your arms out like you're doing a push up. Then push the ball so it makes your butt goin the air. It takes cordination but it works.
These were kind of hard to explain but I hope you understand.