The biggest thing is daily working the social networking sites such as Twitter, Facebook and LinkedIn. I personally focus on Twitter and I've noticed a steady, but modest, increase in listeners to my show. The trick is to take the time to communicate with people. You can't just post stuff on Facebook/Twitter and expect people to follow. I take the time to reply to folks, which really helps build a fan base.
Cross promotion is another, find like podcasts and trade links in your sidebar. Talk about each others shows too.
Go to events that are related to your show topics. Going to blogging/podcasting events is also a great idea. PodCamps and WordCamps are great for networking with folks. Don't forget to make some business cards with your podcasts information, that's the easiest way to give someone you meet your info without them having to write it down.