Free drinks...

... are never free. Because when you know the bartender he expects you to tip out the ass -- at least fifty percent for each drink. Therefore free drinks are discounted drinks. You're essentially using a "you're my boy" coupon. If you fail to tip accordingly you jeopardize the relationship AND the friendship. He may play it off like he doesn't expect anything, but if you call his bluff he'll be disappointed. It's a sign of disrespect. This is the procedure:

-- you order a drink
-- he tells you this round's on him
-- you tell him you'll pay
-- he insists this round is on him
-- you thank him
-- nurse the fuck out of your drink
-- if the next round is on him, be prepared to tip big. if not, the drink will probably be significantly discounted. be prepared to tip big anyway.
-- leave or go to a different bartender

DO NOT hang around and try to get more than one or two free drinks unless you saved this motherfucker from ass rape or unless he's your best bud of all time. In my case, the bartender would have to be Patrick for me to take liberties with him. And even then I'd still tip substantially.

Play it smart.