Most broadcast stations have a public service quota to fill. If your event helps the greater good, creating a short radio spot will not only gain free advertising, but will convey the not for profit essence of your event.
Don’t have a philanthropic element to your event? It’s the perfect incentive to team up with one and give back to your community.
If you haven’t got a heavy advertising budget, then use a 1-to-1 approach on a very targeted demographic. Make clever parternships with other brands and exchange consumer contact details to send emails to them and bring you visibility to a specific audience.
Go to advertising schools and look for talented advertising students recently graduated and partner with them. Recent grads need a portfolio to present to advertising agencies, so they’d be willing to help you in exchange for building their portfolio to get them a job.