My pilates instructor has given me two methods, they work for my body.

Standing method (if you have very long feet or flexible hip sockets, this may not work, most of us are proportional though)
1. Stand with your feet together.
2. Keeping your ankles together, point your toes away from your ankle (so your feet are in a V)
3. Align your ankles with your toes. Your heels should be about sits bone width apart.

Sitting:
Sit down on the ground and kind-of pull your buttock flesh (sorry, cant't think of a more technical term) out from under you. You'll feel your sits bones a lot easier this way.