I'm going to sound like a late night infomercial, but back in June I noticed my stomach felt a little too soft, so I started doing knee raises while hanging off the ground on that device that looks like a chair without a seat. They were hard at first, but now I can do 100 reps at a time, and my stomach is harder and less flabby than it's been in several years.
Perfect for me because I've always hated situps.