Sites like digg.com, reddit.com, stumbleupon.com etc can bring you a LOT of traffic. How about getting 20,000 and more visitors a day when your listing hits the front page?
Getting to the front page of these sites is not as difficult as it seems. I have been successful with digg and del.icio.us (and not so much with Reddit though the same steps should apply to it as well) multiple times and have thus compiled a list of steps that have helped me succeed:

1.Pay attention to your Headlines

Many great articles go unnoticed on social bookmarking sites because their headline is not catchy enough. Your headline is the first (and very often the only) thing users will see from your article, so if you don't make the effort to provide a catchy headline, your chances of getting to the front page are small.
Here are some examples to start with :-

Original headline : The Two Types of Cognition
Modified Headline : Learn to Understand Your Own Intelligence

Original headline: Neat way to organize and find anything in your purse instantly!
Modified Headline : How to Instantly Find Anything in Your Purse

Here is a good blog post that should help you with your headlines.

Write a meaningful & short description
The headline is very important to draw attention but if you want to keep that attention, a meaningful description is vital. The description must be slightly provocative because this draws more attention but still, never use lies and false facts to provoke interest. For instance, if your write “This article will reveal to you the 10 sure ways to deal with stress once and forever and live like a king from now on.” visitors will hardly think that your story is true and facts-based.

You also might be tempted to use a long tell-it-all paragraph to describe your great masterpiece but have in mind that many users will not bother to read anything over 100-150 characters. Additionally, some of the social bookmarking sites limit descriptions, so you'd better think in advance how to describe your article as briefly as possible.

3.Have a great first paragraph
This is a rule that is always true but for successful social bookmarking it is even more important. If you have successfully passed Level 1 (headlines) and Level 2 (description) in the Catch the User's Attraction game, don't let a bad first paragraph make them leave your site.

4.Content is king
However, the first paragraph is not everything. Going further along the chain of drawing (and retaining) users' attention, we reach the Content is King Level. If your articles are just trash, bookmarking them is useless. You might cheat users once but don't count on repetitive visits. What is more, you can get your site banned from social bookmarking sites, when you persistently post junk.

5.Make it easy for others to vote / bookmark your site
It is best when other people, not you, bookmark your site. Therefore, you must make your best to make it easier for them to do it. You can put a bookmarking button at the end of the article, so if users like your content, they can easily post it. If you are using a CMS, check if there is an extension that allows to add Digg, Del.icio.us, and other buttons but if you are using static HTML, you can always go to the social bookmarking site and copy the code that will add their button to your pages.
Here is a link that should help you add Links for Del.icio.us, Digg, and More to your pages.

6.Know when to submit
The time when you submit can be crucial for your attempts to get to the front page. On most social bookmarking sites you have only 24 hours to get to the front page and stay there. So, if you post when most users (and especially your supporters) are still sleeping, you are wasting valuable time. By the time they get up, you might have gone to the tenth page. You'd better try it for yourself and see if it works for you but generally posting earlier than 10 a.m. US Central Time is not good. Many people say that they get more traffic around 3 p.m. US Central Time. Also, workdays are generally better in terms of traffic but the downside is that you have more competitors for the front page than on weekends.

7.Submit to the right category
Sometimes a site might not work for you because there is no right category for you. Or because you don't submit to the right category – technology, health, whatever – but to categories like General, Miscellaneous, etc. where all unclassified stuff goes. And since these categories fill very fast, your chance to get noticed decreases.

8.Build a top-profile
Not all users are equal on social bookmarking sites. If you are an old and respected user who has posted tons of interesting stuff, this increases the probability that what you submit will get noticed. Posting links to interesting articles on other sites is vital for building a top-profile. Additionally, it is suspicious, when your profile has links to only one site. Many social bookmarking sites frown when users submit their own content because this feels like self-promotion.

9.Cooperate with other social bookmarkers
The Lonely Wolf is a suicidal strategy on sites like StubleUpon, Digg, Netscape. Many stories make it to the front page not only because they are great but because they are backed up by your network of friends. If in the first hours after your submittal you get at least 15 votes from your friends and supporters, it is more likely that other users will vote for you. 50 votes can get you to the top page of Digg.

10.Submit in English
Linguistic diversity is great but the majority of users are from English-speaking countries and they don't understand exotic languages. So, for most of the social bookmarking sites submitting anything in a language different from English is not recommendable. The languages that are at an especial disadvantage are Chinese, Arabic, Slavic languages and all the other that use non-latin alphabet. German, Spanish, French are more understandable but still they are not English. If you really must submit your story (i.e. because you need the backlink), include an English translation at least of the title. But the best way to proceed with non-English stories is to post them on where they belong. Check this link for a list of non-English sites.