Gather evidence. Take photos or screengrabs, and post them to Flickr so you can point out how effed-up the situation was. Even better: take video, and post it to Guba. Write down the bus number, the run number (that's the small number at the bottom of the windshield), the date and time, what people were saying, a physical description of the driver, and the driver's badge number.
Getting a driver's badge number can be tricky. It might be covered up by a jacket, or in the case of one of our favorite morning 33 drivers, might have a digit or two simply ripped off of the shirt. You might have to ask them for their number, and that's obviously weird and awkward. But don't be afraid. Just ask. Be polite. Don't get into a fight with a driver like you're a skinhead on Geraldo.