How often do you see people repairing ball marks incorrectly?
I see it all the time. They pry and pry up on that ball mark. I can only assume they do so because that's what they see the pros do on TV (and I'm not sure why the pros do it).
What else should be done to teach people the right way to repair a ball mark?
All the damn time. There's the people who've always done it that way and a lot won't change. Worse yet, there's the people who hands down have no interest in even trying to fix them unless they impact their putting line.
Out of the people who've developed a habit, I think education and some constructive ribbing from other golfers could probably convert about half of them. Best chance is to educate new golfers. I've always thought there should be at the very least a spiel for people who are playing a golf course for the first time.
I usually try to fix mine and at least one more when I have the time. Some of the slower rounds I end up fixing way more than I make myself.