Bret Contreras is one of the best minds currently in the strength and conditioning world. His work on the gluteal muscles has changed the way many people train. He recently wrote two articles on the Long Lever Pelvic Tile Plank and the RKC plank which are both more challenging versions of the basic plank exercise. These are definitely worth trying. There is a video on both articles and I’ve put a youtube video of this as well.