This is one thread the rears its head every few months, with LOTS of different opinions. Perhaps the single best bit of advice I got from a seasoned bicycle messenger in Philadelphia last summer was this:

Make a real effort to alert people, be predictable, and realized that to some extent it doesn't matter what you do; some people just won't understand. There will be accidents.

I've thought about his advice as I've ridden the portion of a MUP I use on my commute, and think he's probably right. I've heard about every variation of an alert you can give and seen about every kind of reaction with little or no relationship to the particular alert. Perhaps the only exception to this has been the three people I see on a regular basis (all riding alone) that use whistles hanging from lanyards around their necks. Whenever, I've seen them blow those whistles as an alert (and they are loud) people tend to stop dead in their tracks. No move to the left and no move to the right. The interesting thing is that about half of them are annoyed that someone blew their whistle at them. I guess even this idea won't work well with a deaf person of someone with their music turned up too loud (of which there are quite a few). So, I go back to the bike messenger's advice.... make a real effort to alert people and realize there will be accidents no matter what you do.