It doesn't matter if your girl is well-endowed with a slap-worthy behind or not. If she doesn't, that can be fixed easily--not with surgery but with these butt bras by Bubbles Bodywear. I kid you not. They exist and they'll transform any sad-looking ass into a perky, bouncy one in an instant.
Even if you don't have a girl, you might want to check their site out...for obvious reasons.
There a bunch of butt bras available: there's the 'Double-O Tummy Tuck' which flattens her muffin tops (but why? Why? They're awesome squeezable areas on her figure!) and pushes out her butt cheeks to fluff 'em up. Then there's the 'Push-Up Adjustable Butt Bra', which comes with an adjustable loop so she can decide just how much of her bum she wants to lift. Although it would definitely be the maximum lift, if it were up to us.
Check It: $Varies