Gotta work that bum! Simple as that. I do lunges at the gym. I usually grab two 40-50 lb. weights (one in each hand at my sides), start at one end of the gym, and alternate putting one foot forward at a time and lunging until my thigh is parallel with the ground. In other words, step forward and bend that knee until your thigh is parallel to the ground. Then sorta pop up in a slow controlled motion, bringing both feet side by side. Repeat with other leg. If done properly, your butt will hurt like hell for days.