Title Tags and Keywords -- Paying attention to the title tags on your pages, as well as the pages that link to your pages (and the link text on the links to your pages), as well as how you use the keywords in your content, help the search engines consider the relevancy of your page.
Keyword proximity is a more important factor than keyword density.
Example: Two snippets of 100 words of text:
Dog food ........................ Dog...............................
.................................... ...................................
................................... ............................food.
Both examples have the same keyword density, however, there is a very different proximity. If you were searching on 'dog food', which content would you say is more relevant to your search? Pay attention to the proximity of your keywords within your copy.
When you are writing content, put your keyphrase in the beginning, middle, and end of your document. If there are multiple keywords (i.e. "dog" and "food"), place them as close together as possible.
The words around links in your content are also important, as they provide "annotation" or context for the actual link itself.
Quality trumps quantity every time with link building. You've either got to figure out who the leaders are, and follow them, or write really relevant content.
Your keywords have supporting terms -- the associated keywords that you didn't select. These are "related terms" and you should use them in your content as well -- to support the keyword terms you are using.
A good rule of thumb is that if you can remove your target keywords from your content, and still know what's going on, you're doing well. So when you are doing keyword research, don't throw away the ones that aren't yours. Use them as related terms.
