Is it really true that it's not what you say but how you say it?
The "Wired" article how to Be an Expert on Anything has advice from Stephen Colbert including using lots of acronyms and abbreviations, not being afraid to make things up and speaking with lots of confidence.
I'd have to add that the cadence of speech is important in fooling some of the people some of the time. The two ends of the spectrum are the quick-talking "it sounds good if you say it fast" method and the drawn out "airline pilot talking on the passenger address system" approach.
You could also try mumbling or talking very quietly, so people catch a few words you say and may end up thinking you know a lot more than you do.
You might try a Socratic method (teaching by asking questions) by not offering any answers or opinions of your own but just keep asking questions that may or may not have much of anything to do with the situation at hand but may make people think you are very deep (or dense).