Stand on one leg and lift the other, then draw a slow circle with your knee — out to the side, back behind you, and around to the front — while your standing leg and torso stay still. The stillness through your hips and spine is what makes this work; if your whole body sways to help the circle along, the moving hip is doing less than it looks like. Keep the circles small enough that you can control every part of them, then let the range grow as your hip opens up. Reverse the direction partway through so you're circling both ways, not just one.