This is the easy way of getting gigs: making friends with people. (Kat is especially good at this; everyone feels like she's their best new friend when they meet her.) Alas, the easy way mostly works if you just play in your hometown, because very few people want to book you on a few days notice, and it's hard to make friends with people from afar. We got the rest of our gigs the hard way: calling a stranger on the phone and trying to sell them on something. Normally they ignore you, or tell you to email them, then ignore your email. Repeat, repeat, etc.
So this guy has a store mailing list, and he sends out an email promoting the show to those several hundred people. He's also the head of a local pagan group, so he sends out another email to another several hundred people. He then notifies all of his facebook and myspace friends, which is another several hundred people.
This is the easy, effective way of promoting gigs. He brought in a crowd of some 35 people, which was a ton of fun and which made everyone some money. Big shows are fun: a flautist came, and we talked her into playing along for a few songs. She was quite good! After that, some guy went and got a bongo drum (they sell them at the shop) and joined in too. He was quite bad, but everyone cheered him on anyway.
Alas, the effective method of promoting gigs only works if you play in your hometown. The hard way is to make a bunch of friends on myspace wherever you go, and post to all the event websites about your gig, and tell everyone you see about it. It's very improvisational and hit-and-miss. We got a lot of people at our second Birmingham show (after the show at the pagan shop) because of two reasons: first, we recruited an accordion player, and she brought a group of friends. Second, we're staying with some well-connected couchsurfers, and they brought a crowd of couchsurfers along. (Thanks, Rishi and Mark!) It's iffy though. Our third show in Birmingham was tiny, because all that stuff just didn't work too well.
So that's what it's like to book and promote shows. Basically, try to make friends with pagans.