I hope you're not really doing sit-ups. If you are, then stop, and start doing crunches instead. For more info on why, go see the
Abs entry in my journal. Also, if you really want to get big, you need to get more protein! Shoot for 1-1.5 grams of protein per pound of bodyweight
everyday whether you work out that day or not. Always get a decent amount of it within 30 minutes of your workout, and keep taking more throughout the day. Also, avoid Creatine, no matter what some of these other jokers might tell you. Tests still haven't shown whether or not it's safe, so it's best to avoid it for the time being.