1) Keep it simple. There's a reason why "Make Love Not War" caught on instead of "Have Meaningful Sexual Relations With One Another Instead of Opting for Nonsensical Violence With an Indeterminate End."
2) Don't bring in unnecessary issues or statements. A sign that makes too many assertions is in danger of being disregarded if people strongly disagree with even one of the assertions. For example, the statement "The Earth is flat" will discredit even the most noble of causes if the two thoughts are paired.
3) Consider your audience. They're in front of you and behind you (and possibly sharing your phone line as well). Including a message on the back of your sign doubles your potential audience. Providing a translation into another language on the back is an excellent way to reach out to a population that otherwise would have been missed.
4) Make sure that people can tell what you're protesting. One local protest against Macy's featured identical signs that all read "Do Not Shop Macy's" with no additional information. It made me wonder why no health care for their employees? Obscenely overpriced clothing? The store possesses uranium? All of the above?
1) "Consider your audience" still applies. Not only will your sign be seen at the actual protest, it may be seen in newspapers around the world. While color is important, some colors will look the same when a photo is printed in a newspaper. A yellow background with black printing provides the best black-and-white contrast on newsprint. Also handy to know for that future courthouse appearance!
2) Rectangles are, like, so 2005. Creativity is key to a successful protest. More and more protestors are incorporating unusual shapes and 3-D elements into their signs. Which means making those dioramas in the third grade wasn't a complete waste of time.
3) Size does matter. Your sign could be explaining the meaning of life, but if you do it in 12-point font on a tiny poster board, it will garner no more attention than those "Drug Free Zone" signs near schools.
4) Use easily recognized symbols cautiously. When was the last time you paused to look at a restroom sign (well, unless you're female, in which case you've had hours to ponder the geometrical intricacies of the triangle skirt)? While recognized symbols have their place, the familiar is easy to overlook.
5) Take advantage of technology. Clip art and downloadable images can be used by the less graphically inclined, and Web sites such as www.anotherposterforpeace.com and www.miniaturegigantic.com offer examples for the printing. Using downloadable images can backfire, though, as protesters in Bangladesh found out in 2001 when their pro-Osama bin Laden signs were discovered to include an image of Bert, from Sesame Street. A little bit of proofing goes a long way toward keeping your cause from being linked with a felt puppet.
Of course, none of the above applies if you happen to be Frank Chu, San Francisco's own ubiquitous protestor, whose sign typically contains messages such as "Impeach Van Buren," "12 Galaxies," and "Guiltied to a Zegnatronic Rocket Society." Despite an incoherent message and lack of any realistically achievable goal, Chu has achieved minor fame and even sponsorship from local businesses. There's something to be said for such dedicated insanity unless you're the president, in which case it's cause for protest.