Define terms of web usability

Search engines use a concept known as link popularity in determing the rankings for competing websites. The idea behind it is simple: If many other websites are linking to yours, then it must have some valuable content. With all else being equal, search engines will give a site with higher link popularity a better ranking than its competitor because the site with higher link popularity has demonstrated that it has content that others find useful.

There are three main factors involved in determining a site's link popularity: quantity, quality, and relevancy. Quantity is fairly straightforward -- the more links the better. More important is the quality of the links going to your site. One link coming from a very important site with high link popularity can be worth more than a dozen links from sites that have low link popularities. Finally, links coming from sites that have content related to yours will be worth more than those that don't. A link from a site that focuses on the same keyword phrases as your site will be weighted much more heavily than one from a completely unrelated site.

Your Site's Link Popularity
If you are targeting highly competitive keywords, then you should definitely put in the time to work on your link popularity. Below are some of the different avenues you can take to improve your site's link popularity. They are listed from the easiest to most time consuming.

Directories
Getting your site listed in directories is one of the fastest ways to build your link popularity. Probably the most important directory to be listed on is DMOZ. It is free and several large search engines including Google, use the DMOZ database for their own directories. By getting your site listed on DMOZ, you'll effectively be listed on Google's directory as well as dozens of other directories that use DMOZ content.

Find potential link partners
Another way you can improve your link popularity is to contact other websites you'd like to link to and offer to trade links. Make sure you personalize your email and include an explanation of the benefits of link popularity. Your request should be professional and give the impression that you put some thought into this.

Identify some positive points in the site you wish to link to and mention them in your email. Point out the benefits their visitors will receive by visiting your site and make sure everything is working on your site. Most webmasters won't want to link to your site if it has poor content or broken links.

Writing Articles for Republishing
One of the best ways you can improve the link popularity of your site is to write articles and allow them to be republished on other sites. There are many article submission directories which webmasters use to find content for their websites.

At the end of each article, you include a short profile about yourself and a link to your website. It's a good idea to explicitly state that your article may be republished only if the link to your website is active.

Final Tips
Avoid Link Farms
A link farm is a reciprocal linking service where everyone participating agrees to link to each other. Also known as free-for-all link pages, there's usually no foundation or relevancy in the linking and it's simply an attempt at improving link popularity.

Most search engines are aware of link farms and will either penalize or ban participants from being indexed completely. The minimal benefits you can get from having random unrelated links directed to your site are simply not worth the risk of getting banned by a major search engine. Writing quality content can do wonders for your search rankings, but if you allow other sites to use the content on the understanding that they link back to your site, it can benefit the both of you. It helps them rank better in the search engines while providing content they otherwise wouldn't have the time or knowledge to write. It helps you because you get more exposure to your work while improving your link popularity.

Use Keywords in Your Link Text
Most search engines take note of the text used for links. A link containing an important keyword phrase will be worth more than the same link with unrelated text. For example, instead of using your URL as the link text, use a keyword phrase.