engine - 10:30 am on May 8, 2008 (gmt 0)

The biggest problem with bought in databases is that the information is skewed by people checking on their own sites.

The dictionary and thesaurus, a book on the subject matter, coupled with sample searches, pulls up the majority of short and long tail terms, for me.

Put your mind in the place of a prospective searcher.
Go for phrases and terms that people would naturally type.