Cheating
There are several underhanded techniques, ranging from the mildly unethical to the outright violation of StumbleUpon rules, that some stumblers may use to promote their sites. In this article, I will attempt to convince you that in the end, the methods described on this website are far better than these cheaters' methods.
Mass Friend Adding
The fastest way to manually add many friends is to visit a person's page and look at their friend list. You'll notice that the friend request button is directly above the list of friends. You can easily alternate between clicking request friend and then clicking a random person from the list of friends.
Unfortunately, StumbleUpon will cut you off at about 25 friends per day, and of these 25, only 1 or 2 are likely to accept your request. To top that off, these friends will not be high quality friends who can help boost your submissions to the top, but merely hollow friends. With a limit of the number of friends you can have in total, you want the highest quality friends you can find.
Automated Friend Adding
There are also some tools available that allow you to enter the URL of a popular stumble page, and will automatically send friend requests on your behalf to all people who like the post. While this might help you find active stumblers who are interested in similar topics as you, the same qualms exist as the method above, with the addition of the fact that this is a violation of StumbleUpon's rules.
Fully Automated Promotion Scripts
There are even scripts available that allow for multiple user registrations, mass friend adds, and direct-message spam of your links. These methods are absolutely unacceptable, and will, without a doubt get you banned and cost you money. StumbleUpon handles spam attacks really well and will not bat an eye at kicking you out. Don't hand a single penny over to these companies. Natural StumbleUpon Marketing is much more effective.
Spam Networks
Worst of all are StumbleUpon spam networks, which are at work full-time registering accounts, building networks, and attempting to launch websites to the top. You can recognize these companies by quantitative promises of the traffic they can bring, with lame-sounding testimonials, and a general failure of having learned the English language. Avoid these companies at all cost, because if they don't get you banned, they will just steal your money.
In the end, it usually pays to play fair. The best way to effectively market a website on StumbleUpon is to be a member of the community, and contribute quality content. Focus on creating a website worth being stumbled, build a natural StumbleUpon network, and you will achieve consistent StumbleUpon traffic.