I remember seeing these as bumper stickers back in the 1980s. I believe this particular campaign began in the 1970s. At the same time there were also those bumper stickers with the phrase, "Will the last person in Michigan please turn out the lights."
It wasn't until years later (hey, I was just a kid) that I realized that this wasn't a promotion of Michigan, but a snarky comment about the exodus of Michigan residents due to the poor Michigan economy.
There was also that jingle they played on the radio and on television ads - Yes, Michigan!
It went something like - Yes, Michigan! The feeling's forever.
That's all I can remember. My only hope is that I have now made that song stuck in the head of each and every one of you who reads this post ;).