One of my favorite exercises (or movements, I should say) is this you see to the left.
It's called a bench hip-pop and it's great for tightening the glutes.
The reason I love it so much is because it really targets all the gluteal muscles and the hamstrings. I usually do it between sets of bench, or other heavy exercises as a way to keep my heart rate up and burn more calories while I'm waiting to do my next set.
Also, since mother endowed me with a nice full butt already, when I load these muscles too much, they grow more than I like.
With this exercise, there's only a small amount of load, but a lot of contraction - which still gets the muscles firing and burning energy, without stimulating a lot of growth.
So, what you do is lie at the bench like you see me doing here with one heel on the bench and the other leg out in the air in front of you. From the floor, bring your hips up until your hips are fully extended and your glutes are well-contracted. Then, lower and repeat that movement over again for as many reps and you like - I usually perform 12 to 15.
You can also do this just on the floor or on a swiss ball. Both are effective at waking up the butt and getting it ready to fire. :)