Ditto on Greensave...we've put their decals on all our cart windshields.
GCSAA also provides similar images you can download for free and laminate.
I think the best educational tool is to put tees, irrigation flags, golf balls, whatever you like, in all unrepaired ballmarks, and take a picture to communicate the problem.....like this:
http://cccpgcm.blogspot.com/search/label/BallmarksOr, better yet, let them play through them(if you can get away with that)!
David H. is right though, success dealing with this problem depends on your clientele. Good luck!