At the bottom of each of my posts now, you’ll see a little green button with a “Share This” link next to it. If you click on that link, you can email that particular post to a friend, Stumble it, send it to your Twitter, put it on your Facebook page, and do all sorts of other things with it (I don’t even know what some of those options are!).
I was trying to add a Stumble Upon button but was unsuccessful (I’ve put in a call for help!), and so this is what I’ve managed to do instead. Let me know if you have any trouble using it.
UPDATE: The Stumble Upon button does actually show up, but only when you click on the title of the post, which takes you off the main page of my blog and onto a page containing only that particular post. Weird.
But, you can still Stumble using the “Share This” button if you prefer. 
