My xc (cross country) coach always told us that you can do sit ups until the cows come in (forever) but you'll never get a six-pack. The secret to six-packs is core workouts, some of which involve those huge exercise balls. It's all very complicated but also running helps get you into shape a ton. Start out with 2 miles and work your way up, you get sore but you get over it. In fact I'm going to xc camp in a couple days for a week so I can see exactly what you do in a core workout. Also ab workouts are good too. You do a variation of things. If you want to go hardcore with these workouts do a 10 minute ab workout (aahhh). What you do is do one thing for 30 seconds (time yourself or get someone to time you) then switch to something different. Some of the things are butterfly kicks, bicycle pedals, crunches, full sit-ups, 6 inches (keep your legs at least 6 inches off the ground but not too high off the ground), leg lifts, pilattis (probably spelt that wrong), and much, much more.