Sometimes our creativity needs a little best. The Overture Search Term Suggestion Tool, Wordtracker, and the Google AdWords Keyword Tool can be that best.
You want buyer keywords...when people are ready to buy and have their wallets out. This would include such things as product names, best product, product review, product for sale, cheap product etc.
For me the best key words for a site is not how long or how short it will be the point of it is how it relate to your sites.
I would advise looking at the keyword demographics of your keywords. This will help you understand which are the most appropriate keywords for your target audience.
I think its a process of somewhere research. Also you have to follow some Hit and trial methods to know what is the best suitable keyword for you. yeah, you can use some keyword tools such as Google Ad Word tool to help you in this regard.
its all depend on your website, choose keyword from keyword research tool, use Google Adwards tool for keyword selection.