You can't. You could lose weight which will eventually make your waist smaller but it sounds like you're already very slim and probably shouldn't get thinner, plus your butt and boobs will shrink too so your actual proportions won't change a lot. If you're thin then doing lots of abs work may actually just bulk you up around the middle as your muscles grow, making you bigger.
The only possibility if you have a bit of a pooch is to try and cut down on bloating, so cut salty/sugary/fatty foods and drink plenty of water (less coffee, alcohol etc). But this won't change your body make up, it might just make your belly look a tiny bit flatter in clothes.