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.

2 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.

11 Never submit old news
Submitting old news will not help you in becoming a respected user. Yesterday's news is history. But if you still need to submit old stuff, consider feature articles, howtos and similar pieces that are up-to-date for a long time.

12 Check your facts
You must be flattered that users read your postings but you will hardly be flattered when users prove that you haven't got the facts right. In addition to sarcastic comments, you might also receive negative votes for your story, so if you want to avoid this, check you facts - or your readers will do it.

13 Check you spelling
Some sites do not allow to edit your posts later, so if you misspell the title, the URL, or a keyword, it will stay this way forever.

14 Not all topics do well
But sometimes even great content and submitting to the right category do not push you to the top. One possible reason could be that your stories are about unpopular topics. Many sites have topics that their users love and topics that don't sell that well. For instance, Apple sells well on Digg and The War in Iraq on Netscape. Negative stories - about George Bush, Microsoft, evil multinational companies, corruption and crime also have a chance to make it to the front page. You can't know these things in advance but some research on how many stories tagged with keywords like yours have made the front page in the last year or so can give you a clue.

15 Have Related Articles / Popular Articles
Traffic gurus joke that traffic from social bookmarking sites is like an invasion – the crowds pour in and in a day or two they are gone. Unfortunately this is true – after your listing rolls from the front page (provided that you reached the front page), the drop in traffic is considerable. Besides, many users come just following the link to your article, have a look at it and then they are gone. One of the ways to keep them longer on your site is to have links to Related Articles / Popular Articles or something similar that can draw their attention to other stuff on the site and make them read more than one article.

16 RSS feeds, newsletter subscriptions, affiliate marketing
RSS feeds, newsletter subscriptions, affiliate marketing are all areas in which the traffic from social bookmarking sites can help you a lot. Many people who come to your site and like it, will subscribe to RSS feeds and/or your newsletter. So, you need to put these in visible places and then you will be astonished at the number of new subscriptions you got on the day when you were on the front page of a major social bookmarking site.

17 Do not use automated submitters
After some time of active social bookmarking, you will discover that you are spending hours on end posting links. Yes, this is a lot of time and using automated submitters might look like the solution but it isn't. Automated submitters often have malware in them or are used for stealing passwords, so unless you don't care about the fate of your profile and don't mind being banned, automated submitters are not the way to go.

18 Respond to comments on your stories
Social bookmarking sites are not a newsgroup but interesting articles can trigger a pretty heated discussion with hundreds of comments. If your article gets comments, you must be proud. Always respond to commends on your stories and even better – post comments on other stories you find interesting. This is a way to make friends and to create a top-profile.

19 Prepare your server for the expected traffic
This is hardly a point of minor importance but we take for granted that you are hosting your site on a reliable server that does not crash twice a day. But have in mind that your presence on the front page of a major social bookmarking site can drive you a lot traffic, which can cause your server to crash – literally!
I remember one of the times I was on the front page on Digg, I kept restarting Apache on my dedicated server because it was unable to cope with the massive traffic. I have many tools on my site and when the visitors tried them, this loaded the server additionally.
Well, for an articles site getting so much traffic is not so devastating but if you are hosting on a so-so server, you'd better migrate your site to a machine that can handle a lot of simultaneous hits. Also, check if your monthly traffic allowance is enough to handle 200-500,000 or even more visitors. It is very amateurish to attract a lot of visitors and not be able to serve them because your server crashed or you have exceeded your bandwidth!

20 The snowball effect
But despite the differences in the likes of the different social bookmarking communities, there are striking similarities. You will soon discover that if a post is popular on one of the major sites, this usually drives it up on the other big and smaller sites. Usually it is Digg posts that become popular on StumbleUpon and Reddit but there are many other examples. To use this fact to your best advantage, you may want to concentrate your efforts on getting to the front page of the major players only and bet on the snowball effect to drive you to the top on other sites.
An additional benefit of the snowball effect is that if your posting is interesting and people start blogging about it, you can get tons of backlinks from their blogs. This happened to me and the result was that my PR jumped to 6 on the next update.