Try doing some lunges and work the part where you are extended and low to return rearward to standing by concentrating on using your glutimus (ass) muscle in the un-lunge phase. Do everything slowly too.
Also, the elipto-trainer thing that many gyms have with a an inclined peddle thing for the legs has a inclination setting around 16 of 20 that is supposed to work your ass. Works for me.
I can even set it for 20 and again, focus on isolating my glutes some and I have buns like granite, not to mention quads like granite.