Maybe not for signs exactly but for things like printed flyers, web banners and forum signatures I think a good type of headline to use is one phrased in the form of a question. For example:
Could you really be a Ron Paul supporter and not know it?
In the case of a web banner or a forum signature it could then say "Click here to find out..." Another example might be something like:
You may think you know where Ron Paul stands on the issues but are you really sure?
And so forth. The reason I think these are good is it forces the reader to ask themselves a mental question. In the first case for example they have to mentally ask themselves: "Could I really be a Ron Paul supporter? I don't consider myself one but I guess I'll at least read it to be sure." Other types of headlines make it much easier for the person to just think "Well, I already read some stuff about Ron Paul, I don't need to read that" and just throw it away or ignore it.
