No matter how awkward this glute bridge looks (and feels), it is unparalleled for isolating your glutes.
How to do it: Lie on your back with your feet wider than hip-width apart. Press through your feet to raise your pelvis towards the ceiling (don't arch your back, we're trying to keep the work in your butt). Gently push your knees outward as you tighten your rear. Hold for one minute.
Cheer style: To add some leg action, try pulsing your thighs in and and out as you hold the bridge. And be sure to keep your weight resting on your shoulders, not your neck!