My advice would be to use Stumbleupon yourself and see which websites you stay on longer. And which you bookmark for yourself (etc etc). And why.
And then apply that reason to your website
Perhaps you can ask people of your target audience for their opinions on the matter.
Edit: I took a look at your website. Even though I was looking for the subscribe button it took me about 10 seconds to find it. Moreover, I didn't really know what "subscribe by email" meant, and therefore I didn't know if I wanted it.
My suggestion: make the subscribe buttons appear right underneath the yellow menu bar. Moreover, say something like "subscribe to this to receive so and so", like weekly tips or something (or whatever the subscriber would get).
Also, from the point of view of a Stumbler, the site is a bit messy and the point isn't evident right away. When I scroll down, it's hard for me to get interested in anything specific.
Perhaps you could use specific pages for Stumbleupon? Like, a specific article? That catches my attention usually.
Also! I LOVE the site's name - ingenious.