From my understanding, podcast creators have to track down and get potential sponsors to sign on (if you're lucky and already have a big audience maybe they come to you). Then you splice in the audio of their ad into your podcast.
Whereas I've thought that an easier model for everyone would be to have an ad service that automatically serves ads into your podcast. Similar to how web site advertising currently works. As a creator you simply tag locations in your podcast where you want ads to appear and the duration of the ads. You also give the ad service a set of key words it uses to serve the most relevant ads to listeners. No chasing money and negotiating payments. Just sign up to the service, tag the audio stream, and provide relevant keywords.
An argument against this way of using ads in podcasts is that you lose control over the content and quality of the ads. I think we've all seen the broad range of quality in ads seen on websites...
The other revenue model is a packaged subscription model through some kind of syndicate. It's difficult to get people to pay for a single podcast, but if you packaged up a few under a small monthly fee maybe keen podcast listeners would be more willing to pay. Could be a good way to capture people who want an ad free experience, but don't want to pay for each podcast individually.
Perhaps I'm not aware and these models have been tried already?