Everyone is responsible for themselves during the ride. You only have yourself (and, occasionally, the police) to answer to. But since the first event took place in 1992, an informal protocol has developed to make the ride safe, fun, and effective.

Here's a quick guide that one rider drew up that explains the most important parts (click for a larger view):

Never block emergency vehicles

We're riding to have fun and make a statement, not prevent sick or injured people from getting to the hospital.

Stay together

There's a reason it's called a Mass. It's important that people in the front don't go to fast, and make sure everyone stays close together. Otherwise, riders get left behind, the Mass can split up, and motorists might try to squeeze through gaps, which puts everyone in danger.

Give pedestrians and public transit the right-of-way

If you're trying to promote cleaner, safer, and less polluting methods of transportation, it doesn't really make sense to annoy pedestrians and bus passengers. And although bikes aren't nearly as dangerous as cars, you can still injure someone in a collision, so keep your eyes open.

Be courteous

The temptation to antagonize motorists can sometimes be overwhelming, but try to be at least polite, and friendly if possible. You won't change anyone's mind by pissing them off.

Corking

If the mass is large enough, it's quite possible that it will be in the middle of passing through an intersection when the light turns. In this case, a few riders should form up in front of the other traffic lanes until everyone has made it through. This is called "corking", and it also requires some PR skills, since angry drivers will have a clear target for their contempt. See "be courteous" above.

Have fun!

The most important rule always comes last.