A trick I learned from a friend while in grad school. When they have football games, walk around the tailgate parties right before the game is about to start, people will almost pay you to take their extra food so that they can get to the game. And it's usually pretty good food...

For better results, go next to the ones thrown by corporations. They usually order too much food and the employees do not care and do not want to bring the food back home.

Oh.. and bring an empty backpack :)