StumbleUpon is a social bookmarking service which requires you to install a toolbar into your browser. If the "stumbler" likes a page, they click the I like it! button to give it a thumbs up. If they don't like it, they can give it a thumbs down.The more thumbs up, the better. And, the more "thumbs down" a page gets, it will be dropped.
Download the Stumble Bar here:
IE:
http://www.stumbleupon.com/download.php
FireFox:
https://addons.mozilla.org/en-US/firefox/addon/138
StumbleUpon uses a different means of sending traffic than other sites like Digg or del.icio.us, which tend to send a lot of traffic all at once. However StumbleUpon will send you traffic over a longer period of time.
SU sends traffic that is targeted to you, based on the other pages you "stumble" and only gives you one resource at a time, unlike the other SM sites that give a big list of links.
SU tries very hard to eliminate spam and has many resources for users to report spam. Reduce your risk by creating content on a wide variety of resources like your own domains, blogger.com, tripod, free hosting sites, etc. Also, create a couple of profiles, using different IPs for each. Note that multiple accounts is against SU's TOS.
Note that you don't use StumbleUpon or other social media traffic to sell to or generate AdSense clicks...Instead, think of StumbleUpon as a link building campaign!!!
To be successful, you must follow these tips:
Have good, intersting content. Lists are not only great for "bombing", they are also great for social media, including Stumble:
18 Keys to Pet Health
22 Truths About the War in Iraq
101 Free Car Insurance Tips
Create interesting headlines...
This is from the legendary Claud Hopkins (1923)
http://scientificadvertising.blogspot.com/2006/02/chapter-5.html
Use Cool Pictures
Your TuelBar contains links to tons of resources for free pictures. In your TuelBar, go to:
Design => Free and Creative Commons Images
And you'll see a long list of places to get great, free pics.
Good Web Design
Your page must be clean and look professional.
Don't submit:
news - (has short "shelf life")
short articles/blog posts
obvious commercial/sales pages
only your own content. It seems you may only submit about 20 pages from a single domain. If you add a lot of content from other domains, this limit MAY increase.
Do:
Submit pages that link to you. Use the SU search engine to find resources about you/your's and be sure to stumble them.
Create a strong profile
Use Digg and del.icio.us,and other similar sites to find good content to submit. Also, use these other sites to help you create content of your own.
http://www.reddit.com/
http://www.mixx.com/
http://www.digg.com/
http://del.icio.us/popular/
Use the Newz Tuel to track blogs in a particular niche. Before submitting a blog post, you may want to add a comment pointing to your own resources. 
Be the first to "Stumble" a page. Make sure you use as many of the most popular keywords/tags as possible. Unlike the major engines, use general keywords.
Popular Tags:
http://www.stumbleupon.com/tag/
most popular StumbleUpon searches:
http://www.stumbleupon.com/searchers.php
Find tag pages that are already indexed and listed in Google.
Step one: Bookmark this page:
http://www.google.com/search?num=100&hl=en&q=site%3Awww.stumbleupon.com+%22Websites%22+las+vegas&btnG=Search
Step Two:
Find:
las+vegas in the URL above and substitute your own keywords, using a + to replace blank spaces.
Step Three:
Use these tags on your own Stumbled pages.
Friends:
Connect with Friends (link in your Profile Page) - Post a link to your "friends page" on your sites, blogs, etc. to help recruit more friends.
Any time you give a page the "thumbs up", that page will be served to friends and fans with similiar interests.
Try to make "friends" with Top Stumblers. They have access to a wide network of people and your time may be better spent making friends with a Top Stumbler or two, than to spend time collecting tons of "generic" friends.
SU has a feature to import email addresses from the popular email services (AOL. gmail, etc) to see if your existing contacts are "stumblers".
http://www.stumbleupon.com/find_friends.php
Use Memberable User Name
Use cool/unique avatar
Add users w/similar interests to your "friends" list.
Join groups with similar interests
Use StumbleUpon Ads:
http://www.stumbleupon.com/ads/
5 cents per click. If you have good content, you can get enough thumbs up so your page enters the free rotation. Good if you'd rather spend money than time.