The Power of Cross-Linking
One strategy to obtain impoved search engine position is to logically divide your content across more than one domain name, and then cross-link your different websites using specifically targeted keywords in the hyperlink text (anchor text). Purchasing and setting up multiple domains can be a great way to increase your online presence, and if you cross-link those domains appropriately, following a few simple rules, you can improve your Popularity Rating or PR for all of them.

Cross Linking and Hosting
It is in your best interest to host each different domain name with a separate hosting service. What this does is ensure that each domain name has its own IP address and is located within a completely different IP address range. This may be important as search engine algorithms can easily identify neighborhood cross-linking, and cross-linking between domains that may be owned by the same company or person.

There is certainly nothing wrong with cross linking websites you own, especially if they are contextually relevant. The search engines are not going to penalize you for using one websites to promote another however, they are linkely to assign less value to links coming from websites that are part of the same community or neighborhood, or that appear to be owned by the same company or individual.

It's believed that some search engines may even ignore 'link popularity' completely if the IP addresses of the linked websites are too close together numerically.

Search Engine Themes and Content
It is important to make certain that each of your different domains is rich in unique content of its own, otherwise you run risk of being penalized for duplicate content or for search engine spamming. It is certain alright, and encouraged, to have pages with similar and relevant content in order to facilitate effective contextual cross-linking between your domains, but you should never mirror or duplicate content.

It is also important that each of your themed domains be able to support multiple pages of rich, unique content, preferably no fewer than eight or ten actual content or informational pages. If any of your themes cannot support at least eight or ten pages of relevant, themed content then you should considered combining the closely related themes into one domain. Search engines like to see a website with content depth and will often ignore sites with too few a pages.

Cross-Linking and Search Engine Themes
It's important to group web pages by 'theme'. For example, if you were developing a website on 'debt', you could prepare content based on a number of debt themes such as; debt consolidation, credit card debt, personal debt, business debt, debt management, credit counseling, bankruptcy and more.

How these different, yet related 'themes' are grouped and assigned domains will depend on a number of factors such as potential search traffic for relevant terms within a particular theme, as well as the level of competitiveness for top search engine position.

Theming and Search Engine Position
Although it's unlikely that all search engines incorporate 'theme' identifying algorithms in the same way, it's believed that most do use 'themes' in some way when indexing a website. What this means in terms of search engine position is that the more tightly targeted your theme, the more likely you will be to appear higher in the search results for terms you have specifically targeted with your optimization efforts. Simply put, the more targeted and 'on theme' your content and optimization, the easier it will be for a search engine to identify exactly what your website is about.

In the case of 'debt', if you target all the different aspects of debt with just one website, then any one theme that you optimize for is going to be diluted by all the others. Look at it this way, if a potential client queries the term 'credit card debt consolidation', all else being equal, you will have a better chance of being found if you have a website rich in content about 'credit card debt' and 'debt consolidtion', than with a website rich in content about 'credit card debt consolidation', 'bankruptcy', 'credit counselling', 'mortgage loans' and 'business debt'.

There are other aspects of 'website theming' that go far beyond the context of a page on 'cross linking'. What is important to understand, is that the easier it is for a search engine to establish a well defined theme for your website, the more likely it is that keywords falling within that theme will be ranked higher.

For more information on 'website theming' go to Understanding Themed Based Search Engine Technology.

Cross-Linking and File Names
Always create your page files with names that reflect the main keywords being optimized for. A web page that is being optimized for the term 'credit card debt relief' should be named something like 'credit-card-debt-relief.html'.

Many search engines index the page name portion of the link tag however, don't get carried away. Assigning web pages approprate file names can help in terms of search engine position, but appropriate in this case also means keeping your file names intuitive and not too long. Use a similar approach when naming image files as well.

The use of hyphens, although not necessary, does make the file names easier to read.

Cross-Linking and Content Relevancy
Back links from a web page on a separate domain that includes your main keywords in the Title tag or Heading tag will always do more to improve your link popularity than a back link from a page that doesn't share your main keywords in the Title or Header Text. This is true for most modern day search engines, such as Google and Yahoo!

This is called 'relevancy', and it's becoming all too important when it comes to search engine position and website optimization. The more relevant cross-linked pages are, the more important to 'Popularity Rating' they become.

Cross-Linking and Popularity Rating or PR
The more outbound links there are on a page that links to you, the less the percentage of that pages total Popularity Rating gets transferred to your page. Any transfer of PR ultimately depends on three things:

Actual PR of the page that links to you
Number of out going links it has
Relevancy (see previous section)
In the case of Google for example, a page with a PR of 8 and 40 outbound links is going to transfer less PR to your page than a similar page with a PR 7 and 3 outbound links.

Cross-Linking and Anchor Text
Most modern day search engines look for pages linking to your page that share your keywords, not just in the Title and/or Header tags, but in the hyperlink itself. This is called anchor text, and the cross-link becomes more relevant when it contains the keywords being queried.

So ideally, you want back links to your pages that share your keywords in both the Title and Header tags, as well as the hypertext's anchor text. This tells the search engines that the page is relevant. It's also important that the descriptive content surrounding the anchor text be on topic as well. Relative keyword terms and phrases in close proximity to the anchor tex helps to emphasis to the search engines that the link is in fact on topic and relevant.

You can read Google Page Rank Explained for a detailed description of how Google's PageRank formula works.

Cross-Linking Within a Domain
Although it is better if these links come from outside your domain (preferable from a domain with high PR and not too many outbound links), Google has been quoted as saying that even links from pages within your domain can improve a page's PR, or link popularity as it is commonly referred.

Link as many pages as possible from within your domain to the pages you are optimizing for good search engine position. Make sure that they all get indexed, and always use your important keywords within the anchor text, title and header tags in order to maximize your results.

It should be noted here that Google has stated that you require at least one link to your Website from another domain name to ensure indexing. Without at least a single incoming link from outside your domain, your submission will likely be ignored and your website not indexed.