When I was a NYC messenger I used these two:
Tip #1 : When you are coming up on the left side of a car, check the drivers left mirror for eye contact. DON'T rely on this of course, but it will give you an indication of at least whether they are conscious of you presence.
Tip #2 : Be your own bike bell. Funny as it sounds-awaken your inner rain man and find something to babble when you are shooting through lines of cars. Again, make people conscious of your presence.
Can't think of any more stuff that hasn't already been said