These days there's a message board for everything under the sun. Knitting, MINI Cooper enthusiasts, fans of a computer game, people who think the Southern United States should secede againthere are more niche forums than there are niche podcasts. Therefore, it should be easy for you to Google your podcast topic and find at least one, and probably several, message boards of potential listeners.
There is, however, a huge catch-22 when it comes to self-promotion on the Net. People who spend a great deal of time on the Internet are always looking for "something else cool" to find and tell their friends about. So if you let them know about your podcast, they're going to jump all over it. Butand this is a huge butif you appear like you're spamming the forums to just run in and say, "Hey, take a listen at my awesome new podcast that is focused on tomato gardening!" then no one is going to care. You are a stranger and you're not there to make the community better; you're there to try to get their attention, and many net-savvy people resent that.
So how do you walk the line? Become a part of the community. Yes, this will be a time-sink, but hundreds of new listeners are worth a little research. Begin contributing to the forum community. If the forum software allows you to create a signature (a signoff message that appears at the end of each of your posts), put your podcast name and URL there. If Mur were to try to promote her podcast on a writers' forum, she would put the following in her signature:
Taking this example further, if someone asks Mur about the signature, she would answer them simply and invite them to check out the site. If this doesn't happen, but she does become a more recognized person in the forum, she might answer a general writing question and follow it up with, "...and I talked about this in more depth in my podcast, I Should Be Writing, show 52. If you have some time, here's the link: http://www...."
People will react more positively to you and your podcast on the Net if you're already one of them. Breezing into an established community to post your podcast is like trespassing.
Derek and Swoopy from Skepticality already spent time on skeptical message boards, so when it came time to promote, the path was obvious. Swoopy describes how they did it: