Nobody has ever asked me to please have their butt droop more and be softer. Ever. So luckily, for those of us who want it tighter and rounder and higher, I have lots and lots of great moves!
One of my favourites is the BOSU Hamstring Curls. It requires extra stabilization, which recruits more muscle fibres and therefore requires more energy (read: burns more calories) and tightens and strengthens you fast.
So, try. :)