A nicely rounded buttock that is in balance with the hips is a desire of many women these days, and because of that you may see a lot of ads for a pill or supplement  to "make you bigger."  Same thing goes for men's supplements. Read carefully...most will say "not intended to treat any condition" as a disclaimer.

Weight gain, exercise (the gluteus is your body's biggest muscle), and weight loss in the hips may all create a more attractive non-surgical curve to the buttock area.  

By far the dramatic results will only be gained by fat injections (a "brazilian buttock lift") and implants.  

Weight loss patients with a lot of sagging may benefit from a procedure called "buttock auto-augmentation" where tissue is transferred and the buttocks are lifted.