Great butt exercises would be leg press, lunges, dips, squats.
Don't forget to hit your abductors, too. My only best way to hit those real good are floor exercises like leg lifts (use a weight on your hip or ankle weights for resistance.) You could try fire-walkers, too. Hitting those abductors can help with your outer hip muscles adding to the appearance of your butt's shape.
Of course, note, that genetics play a huge role in how your butt may appear. Some girls have naturally larger rears than others. Give yourself a huge hug and tell your body THANK YOU every day. It works damn hard for you (and I can guarantee that this particular mental exercise will greatly improve how toned you think you look!)
Original weight in 2002: 220
Original weight when joining Spark 1/8/06: 149
| current weight: 131.0