Bridge

Bridging is the easiest way to create great pelvic stability.

Contract the butt muscles and press your strong stomach toward the cieling, creating a flat surface.

Hold for 5 seconds.  Do 2-3 sets of 12.

Progression:  Straighten one leg so that it stays parallel to the bent leg and try one legged bridging.  When that gets too easy, try 2 legged bridges on a physio ball, then one leg on the physio ball.