When a friend of mine said my butt was really small (she meant it in a good way, but I didn't take it well!) I realized that P90X leg work was not enough to keep my rear high and round(er) (it was looking lower and flatter), so for the last 3 weeks, I've been adding some extra leg work during Plyo X day and Legs and Back day. I have a Soloflex Rockit (kind of a leg press/squat machine) that I use for heavy leg presses. On Plyo X day, I do 1 set of Rockit presses during the "breaks" (then I add on a double break after the leg presses). During Legs and Back, I either do leg presses instead of the back work, or add on 5 sets of presses after the workout.
It seems to be working, as the caboose already seems perkier!
It may just be an individual thing: I seem to need more heavy weight work (especially presses) to keep my butt where I want it to be!