I asked something similar here, and one of the answers was so good, I have to repeat it.
Carry a Cold Steel XL Espada. Nobody will say a word.
A little further on was an observation that people are very polite to somebody with a katana.
Gerber makes the Ridge, a nice little framelock that can double as a money clip. Likewise, the similar size K.I.S.S., P.E.C.K., and H.U.G. by Columbia River. I picked up a Kershaw Crown II to add to my "polite knife" choices, because I like one-handers and black micarta. This is in addition to the thirty or so suggestions that have already been made, of course. My suggestion would be to get one of each of everything, but I like little knives.