In order to get more traffic from web 2.0 sites like StumbleUpon, you might want to tell your readers exactly what you want them to do.
Most of the time, people won’t vote for your articles, not because they don’t like them, but because they forget, and because they don’t think about voting (or submitting to various directories).
I have been viewing various discussions from readers on several Norwegian newspapers over a short period of time, and in many posts (way too many) are the authors telling people to vote for the post if they agree with their statements. Most of the posts seems to end up with the same sentence; click on “vote” if you agree with me.
And posts where the authors are telling people to vote, always end up having many votes. And the more votes they have, the more people will read them.
So, I will give it a shot; vote for this post on StumbleUpon if you agree with me that you should put sentences like that inside your posts 😉
My advice is that you should be straightforward, tell your readers what to do. As long as it’s free and it only takes a few seconds, I think that most people will do it.