Check out a (FREE) whitepaper from Thomson/Reuters on techniques to identify KOL (Key Opinion Leaders) in your industry. I've often used academic databases, coupled with citation analysis and other tools (Dialog's RANK command can be useful for this) to find opinion leaders. Click here
to access the whitepaper summary (registration required to view the entire article). I've also used tools like the fee-based Scopus and its free cousin Scirus to discover KOLs ... Scopus in particular can be a very powerful tool for this.