It looks to me that the ability of signs to speed you up/slow down while hitchhiking is significant. I think they are useful in places where you want to specify your direction explicitly, like for example the ramps. When already on the motorways it gets more difficult. I tend to write the next big city on the sign and/or the motorway number, however it has happened to me that people going directly to my destination did not realize the city I wrote was on the way or they were thinking that I am stopping there. So a thumb might have worked better.
It looks like that knowing what to write and how to write it is a whole craft one has to master. What are your experiences and best practices with using signs while hitchhiking ?