Most Helpful Guy
It sounds like you'd like to have a round butt if you could. So look online for the exercises that will help make that happen and get busy. Also modern dance will help give you a nice booty. Gals that ride bicycles over distance usually have a sweet bottom.
Most Helpful Girl
Wear what you like to wear and don't worry what guys think of it!
Guys like to look at girls. If you're a pretty girl, a lot of guys will look at you regardless of whether or not you have a flat butt. Don't worry:)