(big grin) as a former flat butter... it sounds like you're already in great shape and can handle more advanced exercises. Strap on a heavy backpack and do really deep, with excellent form, walking lunges up an incline until you can't walk any more.
Sprints on stairs at least 2 at a time
I found pistols (1 leg squats to really work glutes more than any other leg muscle, they take time to work up to) http://www.powerathletesmag.com/pages/pistols.htm
You can start by doing them onto a low chair, and then work your way down.
Another one is ..ok this is going to be difficult to explain.
Start with one leg on a chair, bench or sturdy object, and the other foot sort of hanging off the side or back. Slowly lower yourself, using the strength of only the leg on the chair - as far as you can safely go. You may need to hold onto something for balance. Push yourself back up with the same leg.
An easier version of this is VERY high step ups onto an object.
The key to really getting the glutes is to get enough of an angle to really activate them - eg: really low lunges on the incline, etc.
To give your glutes the most growth, focus on training them the most for a while, while just maintaining the other body groups - you can't make everything on your body grow a lot at the same time.