Go to wordtracker and look for a keyword with a good amount of daily searches
Go to Google and enter the keywords (if more than one word put in "") to find the compotition.
keywords with around 500,000 results - good
keywords with around 100,000 results - very good
keywords with 50,000 or less results - excellent.
If this a good method then i have found some very good keywords for niches in the investment and fat loss markets.