Step one is to ignore everything Bike2112 said, except the vague reference to sit-ups not being the only way to get rid of your gut. Maybe you know and do this, but you need to incorporate intense aerobic exercise into your routine of abdominal workouts. Not doing so will build muscle under your layers of fat and you will never see results. But, contrary to the Circus Science that Bike2112 presented to us, you need sit-ups (or other abdominal exercise) in order to get definition there.
My advice depends on what type of equipment you have access to. If your gym has an ab board, use that for your crunches. Do 50-80 or so traditional crunches. Rest for a minute and then do outer-elbow-to-knee crunches to work the obliques. Equipment like the piece pictured below should have a handle on it so that you can spin around and do leg-ups, which reverse the crunch motion. You can also vary the motion of the leg-ups to work larger muscle areas. That has been my routine, and it works well. The leg-ups work your lower abs. You'll find that out the first time you do them.
I'm surprised that you saw results using the routine you used. I used to do sit-ups like those, and they never gave me a broad enough range of motion to be very effective.
