How can you flatten your tummy but still stay curvy?

Planks: Try planks. You hold yourself as if you were going to do push ups but don't go down. Hold as long as you can, then you can work on side planks to define the obliques.

Consider surgery: Liposuction of the abdomen, or coolsculpting might be approaches you could use to spot-reduce your tummy. If you have loose skin, a tummy tuck could work well for you, too. Best to see a board-certified plastic surgeon in your area.