The best way to increase your listenership is to be a guest on another person's podcast. Seth Godin recently said that podcasting is the new blogging. I agree, and if that's the case, then guest podcasting is the new guest blogging. A person listening to a show where you're the guest not only hears the real you and your voice, but you're also getting endorsed by a host they already subscribe to and trust. Plus, that listener already has a podcast app open to easily search for your show. This works way better than advertising, which has a lot more friction when going from initial contact to subscription.