While I understand and appreciate the desire to communicate with others (I was honked at, flipped-off, and yelled at on a Marathon course this weekend), I haven't figured out a way for drivers or riders to see a sign.
On the front, I am hunched on my handlebars, so nothing can be read. On the back, I wear a backpack with supplies, so that is not visible.
I think we just have to accept that people think we are morons, and get on with life. For the few who try to run me off the road because I am "going the wrong direction", I just watch for them and bail as needed. I care what some people think of me, but for people who I never meet, and they don't know my name, who cares?
I wear neon green all the time, so at least they see me.