Write things that would interest the SU's demographic. That said, traffic from StumbleUpon doesn't earn hubbers very much money. SU users usually give really high bounce rates and they rarely click on ads. In fact, this is true of any social networking site be it SU, Reddit, or Digg.
Furthermore, it's HIGHLY discouraged by SU, Reddit, and Digg to add your own content to their sites. It's always nice when someone Diggs, Stumbles, or adds your content to Reddit, but let someone else do it, chances are there will be a much better response.
If you're looking for a great way to get traffic, you'll want to look toward getting traffic from the search engines. HubPages users actually make the MOST of their money from people coming in from the search engines, I know I do!
If you're curious on how to get people in from the search engines, HubPages actually put together and SWEET section on how to do this (and a lot more.) I recommend giving it a looksee!
I highly recommend the segments under "Choosing Topics"
But yeah, it's generally not a great idea to Digg, Stumble, or Reddit (or what have you) your own content.