...are the bands that go on after you and the friends they've brought with them.
There are so few people who actively go out to local shows with unknown artists, it just doesn't happen often enough to reliable. When's the last time you walked past a *real* small venue, heard some music and thought, "Hmm, I'm going to walk in there knowing nothing about the bands and give four hours of my friday night with full attention to everything these bands are doing!"?
Attention is a limited resource. People only have one set of ears. As a musician, you're competing against every act on iTunes. Oh, and you're competing against youTube videos of cute cats, getting boozed with friends, video games loaded with explosions, etc. It's a tough world for a musician.
So who can you count on to start building your music career? Where Everyone playing after you is going to be there for your set, if only just so they get their chance to play. The band's girlfriends at their merch table are bored. The (kinda) close friends who got wrangled into going to see the band are anxiously looking for amusement. Talk to them! They're bored, captive, and probably like the musical genre you play. What more could you ask for?!? (Read this part again and pretend you hear me telepathically yelling).
Go talk with them, schmooze, be charming. Promote.