The main methods I have found to improve StumbleUpon traffic have been:

Rating websites
By getting involved in the community and rating some of the websites I visit helps my website appear for StumbleUpon users who have similar interests to my own, therefore they will be more likely to rate my site with a thumbs up! StumbleUpon is time-based, although not to the same extent as Digg, which means the more votes you can get in a short period of time the more traffic you will receive.

Adding friends
By adding friends you will increase the amount of users who will see your newly added webpages, making them more likely to give your site the thumbs up.

Submit new webpages
Once I have created a new blog post for example I will submit the URL to StumbleUpon using the toolbar, it is perfectly fine to promote your own website providing you don't overdo it. This will help to increase the amount of my webpages I have present in the StumbleUpon database and improve the likelihood of my site being 'stumbled upon'.

Carefully selecting topic titles
It is important to categorise webpages carefully to ensure that it will be displayed to relevant users.

Adding multiple tags
By using multiple tags you can help to attract a wide range of users and by keeping these as relevant to the content as possible will help to keep the traffic targeted.