Taboo Your Words and Wrong Questions together. There are numerous debates over definitions that I'd previously have happily engaged in, but now I just take a glance at them and shrug them off as trivially solvable by tabooing. Applied this into just about everything, from philosophy and Searle's Chinese Room to politics and "is online piracy theft". I feel it has considerably clarified my thinking (and it was my second candidate to the "most useful thing" thread).