What's an effective way for me to lose belly flab without losing any weight?

Plastic surgery: There is no such thing as spot fat loss. You would need to lose weight in general & a certain proportion of that will be abdominal fat. This can be done by eating healthfully, being physically active and making sure that calories burned exceed calories consumed. However you can improve your appearance by toning your abs. To tighten your abdominal muscles consider doing crunches, reverse crunches.