It is probably a matter of loosing that extra body fat. Some exercises that might help the musculature of that area are unilateral leg presses, Lunges, Seated Leg Curls, Lumber Jack squats etc
Also a lot of running can help for that extra definition you're talking about it. I would like to look that up for some more information. A good example is if you compare a swimmer to a sprinter. Swimmers have no butt or definition in that area, whilst sprinters are really defined.
Agreed with all of the above. Lying leg curls good. But wide stance squats do the trick and definitely deadlifts oh and Hack squats. You have to work butt and glutes like crazy. Bodyfat loss helps with the definition for sure. I am still a work in progress on glutes but I had the flattest butt EVER and squatting has made it well much higher and tighter! Squat HEAVY. Contracting your glutes and keeping concious of the contraction all during the reps makes a difference too.
If you already have great butt muscles as you claim, then lowering your body fat should be of prime importance.
But if you're still looking for which exercises to do... I'd say the ones mentioned here are good but seriously... there is one thing that has helped me get a great ass. Not trying to boast... (ok maybe a lil). but go join a demanding kickboxing class.
Kicking drills have given me really solid stabilizing strength in my gluteus maximus (butt muscles).
Out of curiosity, would lower back muscles contribute at all to the "lifting" of the butt or am I just grasping for logic here? But it seems like the same way they say working the pectorals can "lift" the breasts, working the muscles just above the butt area could lift it too... or does it not work that way? =P