I have some good friends I met through concerts.

There are a couple smaller venues that fit a few hundred people that I frequent. When you catch a lot of shows in a particular genre and the crowd's that small you'll start to see some familiar faces at each show. You already have something in common which makes it easier to strike up a conversation.

One band I like plays three nights at this large amphitheatre every year, and you basically have to camp there since there aren't any hotels nearby. So I've also met friends out there since you get a whole weekend to hang out and if you go every year you run into other people who go every year.

Talk in between songs/bands. Ask if they want to hang out after the show. Ask for their phone number to talk later.