Some common design choices are particularly bad. Poorly rendered photographs of the candidate are a major mistake. Is the candidate's face going to be on the ballot? If not, then why put it out there except to gratify their ego and give critics something to draw a moustache on. Look at the poster for Kristina Lawson. If that's the best facial expression you can come up with for your poster, then better to leave your face off. Another common problem is picking the wrong fonts. Complex script fonts which overlap are hard to read. Even worse when you add shadows or outlines or special effects or decorations to them. It also doesnt help to have text in a color that fades into the background, or to have too many different fonts. Look at Jay Ramras' poster from this year's Alaska primaries. he makes pretty much all these mistakes. A number of critics are convinced he lost the election because no one could read his poster or voters were looking for Trey Rampas on the ballot.
Perhaps most of all, don't put things on your poster which people can use to make fun of you. That's not the kind of publicity you want. Tea Party populism has led to a lot of people producing amateurish posters where they take themselves too seriously, but the prize has to go to Hawaii's Ed Justus with his ful-color poster which features a horrifying photo cartoon where he appears with a giant head, plus an unreadable font, a kitschy slogan in a type size too small to read from more than a couple of feed away and a layout which has everything spaced wrong. His sign is so annoying I want to get some just so that I can take them out in the back yard and use them for target practice. Maybe looking like a tourist is a good strategy when you're running for County Council in Kaua'i, but I can't believe the local community doesn't find the cartoonish approach to the campaign somewhat insulting. We'll see in a couple of week if the "lovable goofball" campaign strategy works out or if he has to go back to just being the owner of the westernmost bookstore in the United States.