I think the first thing to do when researching keyword for Search Engine Optimisation (well this is what I usually do) is to determine what goes in the meta tags and that of course depends on the industry you are targeting.
And to that I will do a search in all leading search engines to find out what keywords their users are using to search for the product or services I am trying to sell.
I will then find the four or five top keywords, incorporate it into the meta tags and the main body text and footer where possible and then start building link using the identified keywords.
I am sure other people probably do it differently, some people use tools such as Overture Keyword (
Keyword Selector Tool) tools to select keywords, other use Google sandbox tool (
https://adwords.google.com/support/b...62&ctx=sibling) and some people use Keyword tracker (
Keyword services for professional search engine optimization)