rockzilla

I worked as a bike messenger for a few days until an injury forced me out of the job.
First of all, expect to look deadly traffic in the face and laugh. Waiting for the light will cost you valuable seconds which could be the difference between getting your super-rush package to it's destination on time or not.
Wear all the protection you can find, and ride a very light racing bike, but not one that's too expensive, get something cheap enough that you won't mind replacing when you get hit by a car.
You should know every single shortcut and back alley in your city since the roads are never the fastest way to get somewhere.
The place I worked at paid a certain amount per package delivered. I can't remember the exact amount, but I was taking in $35-$40 on my first few days, and the veterans were easily making more.
It's a very very very dangerous, physically demanding job if you intend to do it well. But there were also a few retired guys that I worked with that took their time getting around. They made very little money, but they just wanted a job that would let them move around outside.

Oh yeah, we were considered 'Independant Contractors'. We weren't taxed on our paycheques, so you may have to set aside some of your pay for the government. We could also write off any bike-related expenses as a business expense.