According to Market Samurai these are the 4 steps to finding the best keywords Relevancy, Traffic, Competition and Commerciality.
The 4 steps do help to put a nice structure around the keyword or market research phase for your web site, however the 15 minutes I spent trying to work out how to use the market samurai tool itself – it just seemed to be a reworking of what you could get from Google Adwords anyway, and not necessarily any clearer than Google Adwords keyword research tool either.
A good indicator of the amount of money advertisers are spending on a  keyword (which should hopefully correlate very strongly with amount of spend by actual customers on the related product) is
Adwords Cost per Click x Total traffic for keyword
Another good keyword research tool
 Google Adwords Best Practises