The first 50 were easy. They all signed up quickly upon announcing that the podcast was coming. Since then it has been a slow growth to 100 and this is how I have been doing it:
The podcast is going to be a seasonal podcast so I have been recording episodes, but I have not been releasing them to the feed. That being said I have several completed episodes. So, I have been releasing them to the email newsletter.
This gives me the perfect incentive for people to join my newsletter. Join the newsletter and get early access to the episodes until the launch.
So, I just send out a once a week reminder even if I do not have a new episode that I finished up saying people can listen now if they sign up for the newsletter. I post this on twitter, because of the developer community, and the Elm Slack, since that is where most elm developers hangout. Every time I send the reminder out I get a new influx of subscribers sometimes only 1-3, but sometimes 10+.