Stimulating the visitors of blog to subscribe to it’s feed, e.g. to become constant readers, is one of the main blogger’s purpose. What can motivate visitors? I think it can be something tasty available only in feed. So I made WordPress plugin FeedOnly which hides from blog visitors the parts of posts you want to show only to your subscribers.
Parts to be hidden for visitors have to be enclosed in tags <!––hide––> and <!––/hide––>. These parts will be available for RSS subscribers only and that will be said to visitors.
Any questions? Ask in comments on FeedOnly plugin page.