There is no better exercise than a lunge when it comes to toning that flabby thing you got trailing behind you! Lunges work your entire lower body and especially tone your butt and your hips. Just make sure you do them right so you don't hurt your knees.
Start in a standing position, step forward with one leg. Bring yourself down by bending both your knees, trying to touch your back knee down to the ground or as close as you can get it. Make sure you lunge down not forward so you don't put too much pressure on your knees. Bring yourself back up to standing and repeat on the other side.