Have you tried using the StumbleUpon URL shortener yet? It's like bit.ly, but su.
pr instead. This way, you don't even have to actually stumble your own posts (which is bad for your blog if you get caught, anyway). Instead, use the su.
pr shortener when sharing the link to your latest blog post (via Twitter, forums, etc.), and people who click the link will see the StumbleUpon toolbar at the top of the page. This will hopefully encourage them to stumble your site, since the option to do so is very in-your-face.
Here's an example of what this looks like: su.
pr/1XNehk (since I'm a newbie at this forum I can't actually post a link yet, haha, so just copy/paste into your browser URL bar).
I've only just discovered this today, coincidentally enough. So I can't personally report on how effective it really is for getting more traffic to your site from StumbleUpon, but from what I've read it works pretty well, so I'm going to give it a shot.