I agree that certain exercises such as squats and lunges (and all kinds of variation of them) will help define the glutes. I’ve also had a few clients tell me that Barre classes have lifted their glutes. So, a combination of these exercises helps along with proper nutrition.
However, genetics also plays a role in how much one can change the shape and size of the glutes (so I agree with Martin’s response on genetics).
I’m casting my primary vote for squats. Lunges are right up there, but I’m a bigger fan of squats for overall lower body development – including glutes. However, make sure the quads are parallel (or slightly lower) to the ground when performing them to get the glutes to really kick in.