Sex.
What? It's true, it'd help!
Well, here's what I'd more seriously suggest to you. Do 10-20 crunches every morning and/or evening. During the day, some time during the after-noon, liberate half an hour and dedicate it to some light jogging, or running if you're used to it. Do that,
consistently, for a few weeks. Your stomach will be 100% flat. If you slack off of course you will have no results. If you do that though, you'll have a sexy flat stomach guaranteed (you didn't mention any handicaps of any sort, so don't flame me if I didn't consider that).
If you REALLY want to get a great workout, like someone else mentioned sign up for martial arts. I know several people who were in Tae Kwon Do, and I've heard Kung Fu is great as well. Judging from knowing them, I'd highly recommend Tae Kwon Do or any form of martial arts for both physical and mental health. You will feel better about yourself, too.
I do warn you though, martial arts (at least most of them AFAIK) are quite intense, and if you're not serious about it, you will be wasting your money and time, and won't make it anywhere. You could also learn to box.

Despite what anybody may think, boxing provides an
excellent cardio workout, works out your entire upper body from the upper abs, lower back (lats?) and up. I know you may not be looking to "buffen up", but if you do some boxing 1-2 days a week for an hour or so, you will lose that extra fat VERY fast, and you'll have smoother more toned muscle definition.
Trust me on the boxing though, I know from experience. It's fun, and gets ya in shape.
-RSI