Originally Posted by JohnBrooking
On telephone poles or other signs, facing the wrong way, of course, where the wrong-way cyclists will see them. Backs of signs might be good.
I was recently thinking the same thing. My slogan would be: "If you can read this, you're riding your bike on the wrong side of the street."
On potential problem: Posting signs or stickers on utility poles or traffic signs could violate local ordinances (although they may be seldom enforced). However, partnering with the city or county traffic engineering office might be a way around this. They might even be able to assist with the printing and application of decals to traffic signs, since they presumably have some interest in limiting this dangerous practice.
On a side note, was there ever a time when riding against traffic was advocated? I ask this because the practice is so widespread where I live, I can't help but think that some folks were taught this behavior at some point in their lives.