Squats!!!  Good squats, go down to where your thighs are parallel with the floor then kind of squeeze your butt muscles on the way back up.  Use a weight you feel comfortable with but try to push yourself a little.  Like Mr. Wendy said, you can only do so much.  But, exercises that work that area may let you reshape your butt to where its got a little more definition than what it might have now.  Once you get the shape and definition it may be just what you are looking for.  Good luck, Greg