Google now provide for advertisements in RSS and Atom feeds, including podcasts. Google's AdSense system allows someone who has a feed to have a text or image advertisement added to the top or bottom of each feed item (bottom is the default and seems a better idea to me). Google tracks who clicks on the ads, charges the advertiser and give the feed owner a commission. I have created one to try it out for my Net Traveller blog.
If this is too much for your readers, you can select ads to display every 2nd, 3rd or 4th item. By default the AdSense system will choose the colors and format of the ads (choosing the ad format can be bewildering for the beginner, so this is useful).
One issue may be with big ads in short postings. I have my feeds to only provide the first few lines of each posting. The idea is that this saves bandwidth: you then click on the link to get the full item, if you want it. But if each small item has a big ad on the bottom, it will get very annoying. Also I suspect there will be an arms race between designers of feed readers who will try to block the ads and Google making them appear.
The most confusing part of the process I found was that the feed ads only seem to work using Google's own Feedburner. That is you can't just put a code in your own feed code, as is done for ads on web pages. Instead you give AdSense the address of your feed and then it gets FeedBurner to create a version of it with the ads, supplied from the server. So your existing subscribers will not see the ads: you have to give them a new feedburner address.
Using FeedBurner might have advantages, if caches the feed. The feeds from my web server make up a lot of the download traffic and if I can get someone elee to do that it might be useful. It might be worth then providing the whole item via the feed, not just a summary.
FeedBurner provides facilities for checking on readership of feeds, but this is a whole new management system I have to learn about.