Foursquare's check-in service has won about 7.5 million users since launching two years ago, many of whom will go out of their way to check in at specific locations in exchange for food discounts and other freebies. Some of the small businesses that have seen success share their advice, including Philadelphia burger joint P.Y.T., which started offering Foursquare specials shortly after opening in August and recently logged its 10,000th check-in.

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