I found a great article on Smahing Magazine that spoke to the task of selling advertising on a website with low numbers. I believe the concepts in this article are the same for selling advertising on a podcast.
2. Ask your audience with free tools like Polldaddy, and Survey Monkey, and Surveys (worpdpress plugin).
3. When you have 500 – 1000 is a good starting point, but if you have less you can still be usable (if you hare hyper-niche).
4. Put Google Ads on your website, and see what ads show up (and go directly to them). Research similar websites and see who is advertising on their site.
5. Have your numbers ready and DO NOT inflate your numbers as it only leads to failure.
6. Approach advertisers in a personal manner. No shotgun marketing.
WP Ad Center All in One Advertising Ad Rotation Plugin
I found a great plugin that not only allows you to input your own affiliate campaigns and banners. You can also sell advertising on your website based on time or impressions (very cool). The advertisers get to login and view their stats (great looking stats). This handles the entire sale (via paypal). You can check it out at wpadcenter.com