There may be certain tricks you can use to help you do this, but in the end it is all calories-in vs calories-out. The hard part is the hunger that results from this deficit.
Actually, it's not that simple. Someone else suggested the book "Why We Get Fat", which is a good place to start. There is reason to suggest that what we eat is just as important as how many calories we eat. I think this book mentions that we don't get fat because we eat a lot, but that we eat a lot because we are getting fat because of the content of our diet.