I was born with my bubble butt. Runs in the family. But, I've been a long-distance runner since high-school. Go to the gym frequently. The best exercises I've found for the ass include;
squats (holding free weights)
lunges (holding free weights)
Lifting weights can, over time, help refine your body. Put the work in and you will get results. Focus on a 'balanced' routine.
Also...
STAIRMASTER!! ...not the pussy stairmaster with the pedals, the one with the never-ending revolving staircase. It's a love-hate relationship, trust me

. It is an AWSOME work out for the legs, and especially the ass.