We repeatedly play with a few other bands close to our sound that have a fairly sized draw so that their fans grow to recognize/like us and come to our gigs. We also promote on our website, twitter, facebook, myspace, and sometimes have people on blogs/ezines/college radio/internet radio plug our upcoming shows for us. Decent looking flyers help, too. (I'm proud of those, because I make most of them myself.)
These are some of our flyers. The last one is for a gig we just booked two nights ago, and I'm still using placeholder text until the other bands confirm. You guys are the first ones to see it lol (not embedding because some of them are pretty big.)
Playing good music should be obvious. So should putting on a good show. You know, stage presence and what not.