Use a much longer focal length. It compresses the picture more and you don't appear as fat. Of course the person needs to stand back further in order to make sure the same image is within the frame that is to be captured. If the person taking a picture of you was using a wide angle lens, you will naturally appear a little wider.
If you have no idea what that means and just had a P&S.
Make sure when you take a picture you zoom in as much as possible. Have them keep stepping back until they see what they want, and then take the picture.
To give you a better idea:
http://en.wikipedia.org/wiki/P...tion_%28photography%29
Look down at the 4 pictures of that yellow thing. Notice how the bottom right (4th image) looks the "Fattest", and the first looks the "thinnest". Not suprisingly, the first is at a much greater focal length than the last shot which is at a wide angle.