Social Netoworks are a good resources to generate a costant traffic on your site. Personally I receive a good quantity of visits on this site from Digg (45% of total traffic). I already spoke about hou to add digg button in your blogger template in this post, and now I explains how to add a StumbleUpon button in each Blogger's post.
Select a button
You can select various StumbleUpon buttons like these:
Add StumbleUpon code into your Blogger Template
First, save a copy of your Blogger template and after, in Modify Template switch into HTML view. Look for this code:
+data:post.url'><img alt='StumbleUpon' border='0' src='http://www.stumbleupon.com/images/stumble7.gif'/>