Size up the audience before playing there. For local venues, a trip to their open mike night should suffice. If you're playing an out of town venue, email friends to see what people are saying, so you at least have an idea of what to expect.
Arrive on time, and not just for the gig--this also means load-ins and soundchecks, which vary enormously, depending on the venue's needs. This is especially important when playing a restaurant or pub whose staff may not be available to help if you show up just as the peak dinner hour begins. If a traffic jam or other sort of mishap breaks your stride, that's a different story. Call the venue and let them know what's going on so they know you're not at fault.
Heed the conditions for every gig you play. If the venue asks for a 45-minute set, don't stretch things to the hour-long mark. If two sets and a break are sufficient, do likewise. This rule sounds obvious, but it's amazing how many musicians ignore it--your performance should leave people wanting more, not grateful that it's finally over.
Check with the venue ahead of time if there's any special announcements to make, whether it's drink and food specials or the next few shows on the calendar. If time permits, contact the next featured performer and ask if they want you to make any special announcements. Taking these extra steps shows a touch of class, and--more importantly--that you care about the venue's business.
Minimize as many technical hazards as possible ahead of time. Periodically check and re-check guitar tunings before taking the stage--nothing is more excruciating to an audience than watching someone struggle with those gremlins for several minutes between songs. If possible, bring a portable mixer onstage to allow greater control and flexibility over the sound. Make sure all equipment, from guitar cords to amplifiers, is in good working order.
Make every beat, every lyric and every gesture count. Whether the venue is half-full or packed, don't let your focus waver. If someone seems particularly appreciative of your efforts, make a point of establishing eye contact with them. Build a rapport with well-chosen anecdotes about the material that you play--coffeehouses tend to attract an intense listening crowd who appreciate those little details.
Read your audience as you go. Ditch lengthy pre-song intros or extended solos if the crowd seems to shift in their seats or traipse off to the restroom. Different rules apply, obviously, for gigs that go well--but, overall, nobody lost points for sticking to a "less is more" aesthetic. This is especially important in venues that you play for the first time, or supporting other performers on a multi-band bill.
Arm yourself with well-chosen cover versions of other people's songs. This doesn't mean that your own songwriting has to take a backseat, but there's nothing wrong with showing an audience the reference points behind your artistry--which are bound to incite some curiosity on their part, or even a mass singalong or two.
Your audience's experience shouldn't stop once you walk offstage and sit behind the merchandise area. Nobody should feel like you're shaking them down for every nickel--if someone wants a dollar or two off your latest release, give it to them. Throw in freebies or unusual one-off items for every item that people take home. Small gestures build positive word-of-mouth, which is the best sort of promotion.
Clean up your stage area and don't forget to tip the bar or waitstaff after the night ends--they'll appreciate your recognition of their hard work, since your own situation isn't terribly removed from theirs. Send a thank-you card or note to the venue that hosted your show, even if it didn't go as well as you had hoped. This kind of attention to detail does make a positive impression and may lead to something better down the road.