The best exercise to tone your butt is power walking, because walking most effectively uses the large muscles of your body: the glutes, the quads, the calf muscles and the back of the thighs. Walking burns calories, and you can tone your butt all you want without seeing results if it's covered in too many layers of fat cells. The key is to first reduce the amount of fat on your derriere, and then let what's left do the walking.
Pilates people have a nice move for further toning: they call it swimming. You can lie on your stomach with your legs flat on the floor behind you and arms stretched forward, held close to your ears. At the same time, raise your left arm and your right leg (don't bend the knee), then lower them and raise the other opposite pair of limbs, then lower. Get it? You're "swimming"! But if you have lower back issues, don't do it: just keep walking.
One of my favorite rump-raising exercises starts on hands and knees (on a mat, or the floor will hurt your knees). Raise one leg into the air, keeping it bent at the knee, and lift it so your knee comes about even with your derriere; hold for a couple of seconds, then lower a few inches and raise again. Do 25 reps and then do the other leg. Basically, you're pumping your leg in the air, using its weight for resistance. This exercise targets the back of the thighs as well as lifting and tightening the rear end.
