I've had that problem my whole life! They called me "brick ass" in high school....
Anyway, the most effective exercise I've found to add a little shape is really heavy lunges. There are a ton of different ways to do them--i.e. w/a barbell, w/dumbells, one leg at a time, alternating legs--I've even seen lunges where you "walk" the perimeter of the gym.
My preferred lunge is on the Smith machine. Usually I take a step or one of those little wooden blocks you so often find in gyms and elevate my post leg on it. Then I work one leg at a time at maximum weight, which helps keep the tension on the muscles at all times. I do 3 sets of 8-10.
Bear in mind this is in addition to heavy squats (also done on the Smith machine) and leg press. But it's definitely given me some curve in the rear.
