As Mibs said, don't get too technical. I learned that supporting computers and shit. Figure out what the problem is, figure out how to solve it without causing a state of panic or doubt, realize that the client thinks his way is the right way, and don't argue. When I get into a situation involving someone not allowing me to do my job the way it is supposed to be done (because they want me to do it their way; often the wrong way), I tell them that their computer can stay broken until they let me do my job. When they complain, my boss tells them that I'm the IT guy...not them.
You can't rationalize with irrational people. Defending your argument is not worth the shit storm.