http://www.amazon.com/exec/obidos/tg...books&n=507846
I started to write a long expose on this but having suffered this myself from time to time, I found the above book a good guide to being tactful in situations where you do not wish to make the other person feel like an idiot, even when they are being an idiot.
I haven't read it in about 5 years and I really need to re-read it. I've read it twice already, but its easy fall back into old habbits.