OK, since the previous posts were clever enough to answer "either/or" with "and also...", I'll add one of the more interesting new theories I've come across. As a disclaimer -- I have no experience with this myself.
But there was a lot of press a year or two ago about a Berkeley psychology professor, Seth Roberts, who supposedly, after intense experimentation on himself, discovered the secret to regulating appetite -- it has to do with occasionally drinking small amounts of sugar water (or taking spoonfuls of oil). He has a whole theory around this called the "Shangri-La Diet," and a number of fervent supporters. I would be inclined to dismiss this out of hand if I knew fewer extremely skinny people who drink a lot of sugar water...