Obviously you can select an episode, Control-click (right-click) it, choose Show in Finder to open the host folder, and drag old episodes to the Trash. But I like automatic methods and use Noodlesoft's $22 for this kind of thing. Hazel is a preference pane that performs automatic actions on folders you designate. I've created a Hazel action that uses my Podcasts folder as a source and trashes podcasts older than 10 weeks. Using a combination of Hazel and iTunes' Auto-delete feature, I've managed to clean out the cruft (and that cruft will continue to be cleaned out even if iTunes decides to stop auto-deleting my podcasts because Hazel is ever-vigilant).