I just thought of an interesting idea.
The standard RSS icon is available in a 128×128 pixel size. Using Photoshop, you could reduce it to 125×125 easily, and add a legend to it. Put the name of your blog, or something describing your blog (in my case, “Webmaster Resources”) over the bottom of the icon. Make it visible, but don’t cover-up the antenna symbol. (I’d give an example, but I don’t have a copy of Photoshop on this computer yet.)
Next, purchase a 125×125 ad slot on another blog, give them your ad creative, and tell them to link it to your RSS feed.
What do you think? Would it work well? Obviously this would work better with FeedBurner feeds, since clickers will get a list of recent posts. Some bloggers could reject your ad though, out of fear that viewers would confuse your ad for their feed link.