The question is, what tools are available to do keyword research. And how do you make better use of the tools?
I have collected a list of tools and information in my web links: Search Engine Optimization
Some factors to consider are:
Related search phrases: After you spend some time with these tools you learn that there are different strokes for different folks. Which means that one person might look for "fuel efficient cars" while another might look for "best fuel economy". A good thesaurus is helpful, to an extent. The research tools tend to provide lists of related search phrases.
One strategy to follow might be sprinkling the alternative phrases on one or more pages in your site.
Search frequency: This is a count of the frequency the search phrase is used. The more common the search phrase then perhaps the more lucrative? Clearly if your site can rank highly for a common search phrase, you'll draw a lot of traffic.
Competition: This is a count of the number of sites providing pages related to the phrase. The more sites on a particular phrase, the harder it will be to rank highly. Perhaps.
A simple way to measure the competition is with Google. Go to Google and enter the keyword string with quotes around it, telling Google to give you an exact match. At the top of the search results Google prints out the number of pages matching that phrase. That count is the competition for that phrase.
Keyword effectiveness ratio: Clearly there is some kind of tradeoff between a high traffic phrase, and one with few competitors. It's probably more effective to write articles covering keywords for which there are few competitors. At the same time if that keyword has very few queries there isn't much traffic to draw on.
Cost per click: This shows potential earnings from targeting a given search phrase. The higher an advertiser has to pay for the keyword your page offers, the more you earn.
Some keyword research tools, considered
Overture's Keyword Selector tool: You enter a search phrase, and it returns a listing of related search phrases along with the frequency those phrases have been used in searches on the Yahoo search engine.
Keyword suggestion tool: You enter a search phrase and it returns a listing of related search phrases along with the frequency as reported both by Overture (the above tool) and Word Tracker.
7search.com Keyword Suggestion Tool: You enter a search phrase and it returns a listing of related search phrases along with the frequency as reported by 7search.com.
thesaurus.reference.com: Allows you to make thesaurus and dictionary queries.
NicheBot: You enter a search phrase and it returns a listing of related search phrases, along with the frequency, the count of competitors and some ratios giving supposed effectiveness ratios for targeting a given phrase.
Google Adwords Keyword Tool: You enter a search phrase and it returns a listing of related search phrases, along with the frequency and the cost per click. However rather than reporting actual numbers Google shows a normalized ranking on a 1-5 scale.
Keyword Workshop: This is a web site offering a wide range of tutorials on keyword research, as well as reviews of even more keyword research tools than are covered here.
Keyword Discovery: compiles keyword search statistics from all the major search engines world wide, to create the most powerful Keyword Research tool. Supposedly. One feature is to help you find searches based on speling misteaks.