They should catch the eye so that everybody who sees them reads them. Make sure the lettering is large and clear. It is better to have one big clear simple statement on a banner or placard that people can read and absorb in a glance, than have a paragraph that only those who take the time to stop and read it will get your message. This is especially true if there is any mainstream media coverage of the banners and placards. If they do make it into television, for example, an obscure message will be lost, while a clear one can be read in 2-3 seconds.
Graphic images are also good if they are clear and colourful.
Make sure posters are clear and can be seen from a distance. They should be put in places where people will see them as they are normally walking along. This may sound obvious but many posters end up being placed too high, or too low, or at an angle that makes them hard to read. The first line should be bigger than the rest and should reach out and grab them. Then as they walk closer they will start to read the smaller print.