Promotion can take awhile to build interest for a concert. It's important to start early, such as a few months before the concert. Also, if you start promoting a couple weeks before a concert, people might already have plan for that evening. If you reach them earlier, their schedule might be more open and they can choose the concert for their entertainment that night. Continue to promote all the way until the concert happens.
Who are potential fans of this band(s) playing at this concert? These are the people you should target. It's not to say you don't want to promote to other people, but these people are your best chance for getting people to attend. One way to promote to these people is to pass out flyers for the concert at concerts for similar bands. For example, if you're promoting a rock concert, go to other rock concerts in the weeks before your event and pass out flyers. Talk to people and say, "If you like the band playing tonight, you might like these guys, too." Put flyers in the areas these people shop, such as record and clothing stores.
Make sure your promotion has the necessary information, such as the names of all the bands playing, the time of the concert, the cost, where the concert is located (the venue and the city) and any other important information, such as if there is an age requirement to attend. The more knowledgeable of the concert and its detail, the more likely someone is to attend.
It's important to promote both online and on the street. Online, take advantage of social websites to inform people about the concert and try to convince them to attend. As well, mailing lists can offer a way to promote concerts by sending regular emails, informing people of the concert. On the street, put up flyers in locations where people can easily see them. Make the flyers stand out, so people are compelled to look at them. Pass out flyers at other concerts or leave them under the windshield wipers on cars to further try to promote the concert.