When you join Stumbleupon it is a good idea to give yourself a good profile, with a photo and a good range of interests ticked, as users who are involved and visible on the site find it easier to get traffic to their own pages. Build up a network by clicking on the link to the details of people who post information that corresponds to your interests, and then you can share pages of your own. It's not helpful to 'like' your own website, though - Stumbleupon regards this as spamming. A better way is to get other users to 'like' your pages. You will soon build these up by networking as mentioned, plus some people get friends to join and post links to their pages as well (you can stumble different pages from the same site, but again don't just go to a friend's site - or get them to go to yours - and simply click on every page of the site in a row, as this is also seen as spamming. Spread your stumbles over several days.
You can find more ideas here.