External linking is an essential part of an effective SEO strategy. It is also, arguably, one of the most challenging aspects. Take a look at five common mistakes SEOs make when developing a linking strategy.

#1- Judging a Site's Authority by PageRank Alone:
Good link building strategies aim at getting a links from a diverse array of websites and as with most things in life, all websites are not created equal. How do you judge a good website?

The answer is authority. PageRank is a popular metric used to judge a sites authority. However, pagerank can be misleading. It is unclear exactly how a sites pagerank affects a sites position in the SERP. PageRank is also only updated about once a quarter, if that. Don't completely ignore pagerank but be aware of its limitations and use additional factors to measure a sites authority. SEOmoz has two helpful metrics, mozRank and mozTrust. Also look at how humans view a site, start by looking over the comments, user ratings or anything else that will tell you how internet users judge the site.

Here are some additional questions to ask yourself:

• Does the site feel like a trusted site?
• Does the site have readers who take the content seriously?
• The site has authority, but is it a relevant site to my website/client/topic?

#2 Building Links with Only One Keyword or Keyword Phrase:
The goal of successful link building strategy is a natural link profile. If 1,000 people link to your site it's probably safe to assume that they won't all use the same keyword phrase.

While most SEOs may know this, if you aren't conscious of it with each link you will, like most SEOs, use your primary keyword as much as possible. Keep track of the anchor text that you are using, make sure that there is a mixture of anchor text containing: brand name, website address, secondary keyword phrases, click here etc. This also opens up the opportunity to link with secondary keywords to interior pages of your site.

#3 Aiming for a Billion Links!
How many links does a site need to be competitive? Your competitive research holds the key to determine this. If your top competitors average 1,000 links, aim for what you need to be competitive. Set your goal number of links 2-5 percent more than your competition.

This way you can maximize the efficiency of your SEO activities and properly scale your link building techniques. If you don't set a goal number based on competition you could be spending too much time on link building instead beefing up the on page optimization or focusing on social media.

#4 Only Link to Your Homepage:
Attract links to a variety of pages on your site. If you have a good internal linking structure linking to any page will increase the overall pagerank of the system; strengthening all the pages in your website. Additionally, it will help you rank for more keywords and drive more traffic to your site. It also forces you to think of the user; will linking to an internal page provide more value to the target audience for the content.

#5 Analyze Competitors Backlinks Who Don't Rank for your Target Keywords:
Often the top five competitors in an industry may not be the top five sites in the SERP. Conduct a search for your terms, analyze and review the linking activities of the sites that show up in the top five positions.

For the overall marketing direction it is good to review the on and offline marketing activities of the industry leaders. But only through focused, competitive research based on the most visible sites in the SERP, will you get a sense of what linking activities your site needs to be competitive.

External linking is constantly changing, but if you focus on building a balanced link profile that helps Internet users find the valuable content on your website then you will come out ahead in the long run. Kelvin Newman wrote a link building book called Clockwork Pirate that offers a complete list of linking techniques and the best way to approach them.