Targetted "crunches" as opposed to sit ups, pilates can help tone your stomach as long as you avoid fast food bloat and keep your diet balanced.
If you belong to a gym there is a butt blaster machine you can use to really firm and tone your butt and accentuate the muscles. You can mimic the machine by going on elbos and knees, catching a small hand weight in the crease of your calf, flesing your foot and pulsing/raising-lowering the bent leg.