Ahh…. aside from free sex with the hot bartender or hot customer, this is probably the most desired item on the agenda of just about every bar patron on the planet.
How do you score free drinks? Don't ask for one! That's Rule #1 and by far the most important. Under no circumstances go out expecting free anything at all – even if you've gotten free drinks there 100 time prior. Always be prepared for and expect to pay in full. Nothing will turn a bartender off more than asking for a free drink. You've instantly painted yourself as a self-righteous, pompous, cheap, non-tipping ass. You've blown it and will almost surely never get a free drink. The only way to extricate yourself from this situation is distraction – a.k.a, massive tipping and getting out of there. Come back another day to do battle and hope that your bartender has forgotten your bar faux pas. If you've tipped massively enough, he/she will remember the cash and not the stupid request. Heck, you might even get the first one or two free. Learn from that idiotic mistake.
Rule #2. As in my other posts, your objective is to project yourself as that problem free, non time-sucking, heavy tipping regular. If you do that, you're pretty much guaranteed to score free drinks – sometimes on the level of more free than not. See where this is going? You can actually wind up spending less over time and making your time at the watering hole much more enjoyable. Even in establishments where you've broken Rule #1 and asked something retarded like "what's the buyback policy here?" you can recover. Your bartender will surely answer something like "we don't do that here." Translation = "What a dick. He's never getting a free drink."