Ask them if they know this so well, then why the hell are they continuing to do the wrong thing?
There comes a point in almost every type of thing (guns, cooking, driving, etc) where you've learned enough to know which rules are overly cautious and which are necessary, or which rules are in fact meant to link to other behaviors. On the other hand, after some more time, you realize that many of those rules are not overly cautious, but in fact make good sense.
Many people skirt rules simply because they see other people doing it. They don't want to put forth the effort, and quite frankly, it doesn't seem "cool". Deep down, everyone wants to fit in, and they don't want to be the only one demanding that people lock their slides back or put their ears on before going into the range.
Lead by example, and get as many others as possible to do the same. Don't slip up, not once - practice what you preach in every single instance. Most successful professionals of any kind have gotten where they are by not only practicing, but practicing good habits and form - whatever it is - shooting, weightlifting, bicycling, speaking, etc. There are exceptions, but those people (Mike Tyson) usually don't last long.
Basic rules exist for a reason - because it represents the most sustainable level of optimization available in a certain context. These have typically been developed, tried and tested by many who have come before, and they are rarely wrong (although amendments can occasionally be made).
