I tell myself that the only way to get rid of this fat on my body is to exercise. If I don't do it, then it's just going to stay on there one more day. Right or not, that usually motivates me enough to start working out. The longer I wait to exercise, the longer I wait to get the body I want.
While I work out I say things like, "the harder I work, the more fat I leave behind" and mentally picturing myself walking or running away from blobs of fat. (Wouldn't it be nice to really be able to do that?) I also tell myself that sweat is just my body's way of getting rid of the melting fat. hehehe I know that isn't true, but it's kinda fun to think like that.
Some people use phrases like that. Some use posters or pictures of what they want to look like. The trick is to find something that is more important then not exercising (whatever it is you would be doing).