Doesn't everybody want to be thin? Yep. Even me. The thing is, REAL thin isn't pretty in person, at least not to me. Supermodels look one way on paper, and another way in real life. That's why they starve their bodies, so they don't look so fat on camera. You see, on camera, things look a bit different than in person. Cameras tend to make us look a little heavier than we actually are. 5-10 lbs heavier in most cases. So one thing that is requested frequently (especially by women) is"Can you make me look thinner?" The answer, of course (thanks to Photoshop) is "YES!" And the best part is, it can be done in such a way that it isn't obvious.
The weapon of choice for 'putting your clients on a diet' is the Liquify tool. It lets you push, tuck, pull, and nip just about any bulge or roll you can imagine. The trick though is to use the FREEZE and THAW tools, and use a large tool size. Liquify basically lets you play with digital putty. It's not like the smudge tool, which mixes pixels, it's more like the Warp tool except it is focused on your brush area - plus you can freeze pixels to keep them from moving. The Liquify tool is located under the Filter menu. Just select a layer you want to 'slim down' - click 'Filter' and then click 'Liquify'.
Now all that's left is to examine the image and determine what would make it look more natural - and thinner. Use the tool to push, tuck, pull, twirl and shed pounds as fast as Steve Vai plays guitar. Don't over do it. It won't look realistic. You don't usually want to lie (though you can). Most often the goal is simply to flatter, not fabricate. I find that, more often, clients are happier when they look 'better' without looking like they just got out of high school (although some would like that too!)
Sources: http://porterphotorepair.com/ Article: http://ezinearticles.com/?Photo-Restoration---Making-Fat-People-Thin---Orat-Least-Helping&id=1856182