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This is a fairly frequent topic, and there are many different approaches you might take. Likely you will need to find an approach that works best for you.
This could be anything from online tools like Wordtracker and alternatives [webmasterworld.com] to perhaps less conventional approaches like a dictionary and thesaurus [webmasterworld.com]. If a list of tools would help, there are quite a few services in the thread Evaluating KW tools [webmasterworld.com].