Sorry if that's a vague question - I'm not sure how to phrase it.

Basically what I mean is this:

My band gigs quite a bit around London. Some times a few people come to see us, some times barely anyone. Very occasionally quite a few of people come to see us. Once or twice we've done gigs to decent-sized crowds and they've gone really well but we never really keep track of who comes to see us and don't really have a way of telling people who've enjoyed us but might not know what we're called who we are or when to see us again.

Sometimes I have conversations with people after sets where they ask the name and say they'll check us out/come to more shows but I'm guessing often they forget the name by the end of an evening's drinking and a morning's hangover.

So what I want to know is what means do other people in bands have for trying to keep hold of people once they're intersted and get them coming to more shows?