I made a career out of this in college. It's partly why I got into PR.
Vendors, media account execs and others always want to take you out for lunch. I never pass up the opportunity if the food is free!
ETA:
One of roommates from sophomore year and I would go to Publix on the days where they were giving out their food samples. We'd fill up on those so that we wouldn't have to pay for a meal.