Even with cars zooming past, potholes, and doorings, I don't often feel unsafe biking around the city. I think the one reason is that I must be one of the loudest bikers you will ever hear.
Anytime I feel at all unsafe, I do my hand-free cab whistle (which usually echoes to give you an idea of the volume). I love the stunned look on most motorists faces, as they look up and down the block trying to figure out who made that noise. This really works. I rarely have problems with people knowing I'm there or being considerate to me.
The link that will teach you the whistle, for free of course, is:
Plus, as I mentioned before, it's fun, less hard on the voice than yelling, and can REALLY grab some attention when you need it. Plus the sound is more polite than a car-horn, which is, in my humble opinion, always annoying.
I'll be available to coach this on the forum. If anyone has any trouble, please feel free. Also, if a few other people would step forward who would be willing to teach, I think we could make a few people's commute a lot safer.