How to mount posters, bills or placards on a bicycle
The most practical approach we found for larger posters was trailers. Like this They're somewhat inclined to fall over in the wind so you have to be a little cautious, but the group who were using them lent them out to a range of people without any problems (other than getting the trailers back afterwards). You can attach the same A frames to a loadbike if you have one. 

Up to about A3 you can strap to your back, ideally to a backpack, or to the sides of a rear rack. I've either laminated posters or printed on coreflute, then stapled them to a wooden frame and cable tied them to the bike. The problem is that you have to print large for people to be able to read it, and that means "BIKES GOOD" rather than a paragraph. The graphic below I stole from a guy in Sydney because it's IMO good - simple, easy to grasp and direct. You could leave the "critical mass" off if you wanted to, obviously.



If you want to attach slogans to cyclists it's easier to do it on a T shirt. Critical Mass have a huge range of advocacy shirt designs and places like the Chicago Flyer Exchange have a range of artwork ready for you to use. People will generally pay you for the shirts, so you can use them to fundraise a little as well (cost of a printed shirt is usually a lot less than people expect, so just rounding up to the nearest convenient price usually results in a cheap shirt and a small surplus).