StumbleUpon is, in my opinion, one of the best traffic sources for a site/blog and, more, it is an addictive tool to discover new sites in areas of interest you choose (my SU account is here, add me as friend so I can see what interests you :) )
I have stubmled yesterday on this page that shows a series of answer from top stumblers to two questions:
1. What do you usually look for to stumble (article, video and podcast)?
2. What do you advice bloggers to do in order to increase their chances of getting stumbled upon?
It is interesting to see what the top stumblers say:
– it is important to participate in the community: vote on other pages, discover interesting pages, join groups, network with other users with similar interests;
– have a clean design to your site;
– make the content as readable as possible: stumblers have to be “caught” in your content because another page, maybe a lot more interesting, is at a click of the mouse away;
– catch the attention of the stumbler: use images, use an interesting introduction;
– do not spam other users requestiong votes.
There are more ideas in the responses of the stumblers, but basically: entertain and offer interesting information.