If you are looking into them, I highly suggest getting one that is a one piece, that goes from your shoulders to your legs. I have a couple that are 2 pieces, as in one for upper body and one for thighs, and hate it. The upper body one rides up basically. The leg part is okay. But if you want slimming everyone, go for the full piece one.
I don't remember the brand name, but I wore/wear one that looks like a swimsuit when I wear dresses sometimes because I have this little pouch of fat on my lower abdomen that just will not get lost. They work well, and I agree with the above answer, don't get a 2 piece one, they suck!