To use Google Keywords tool, you should be logged in to get decent results, writes Malcolm Coles (@malcolmcoles) in his post Why you shouldn't use Google's keyword tool for SEO. He offers other tools he says are better.
Some people use Google's AdWords tool as a help as well, but there, too, you need to have some sophistication in understanding it. While the suggestion tool seems straightforward, offering terms that people search for and offering volumes and cost, it's useful to be a bit skeptical, and suspect that Google is offering you as many words as possible to try to get you to buy them. The more you bid on, the more they make, of course. I'm not saying they're dishonest, just that you have to apply some brain power. For example:
- Is past volume an indication of future volume, at least in your target audience for time period you want?
- How much competition is there for the word or phrase they're suggesting?
- Do you have content that will match the word or phrase well and give you a good score in Google Ad words, and therefore help keep your position high and cost low as people click through?
Coles, whom I was referred to by @PerfectMarket, writes that for SEO purposes "I tend to use a combination of Google Insights, Google Autocomplete for web searches and Google Autocomplete for News." He goes on his post, linked above, to describe issues and link to other tools.