I'm working my way back to "high and rounder". When my butt is really out of shape, it is low, flat and mushy, and when I'm in the shower, I don't feel the roundness of the butt cheeks.
Right now, it's higher and rounder (I can hold a pencil betwee my butt cheeks without flexing, and I feel the round muscle when I'm in the shower).
At it's best, it was because I did a lot of lower body work from The Firm (leg presses, various types of hover squats, floor work) and I think step aerobics help as well. I find that if I do too much kickboxing vs. step, my but gets flatter (not what I want!) but that may just be me!