Our band booker does all this stuff in January when there are no gigs, it's dark outside and nobody is doing anything else. She gets a list of local festivals and specialist ones that cater for our type of music. Apparently, you need a high quality video of you playing, so we got one of those (a student did it for £250 and was amazing, so we now use him to work for us professionally where he earns substantially more), and some good promo shots. A short band profile is useful too, apparently.
Then it's a numbers game. As soon as you've been at one festival and done a good job, things pick up from there.
Remember to be respectful (I'm not suggesting you wouldn't, but you know what some bands are like!), and give them time to do their work and select the right bands for the audience.