Teaching your students the importance of holstering properly starts with qualifications and basic skills. How many times do you or your students look to your holster when you re-holster? Do you look at your holster when you are drawing your firearm? Probably not. When I watch officers shoot weather it's qualifications or during a course, I see many officers looking to their holster when they re-holster. I think this is more from habit than anything else. Once I or other instructors address the issue, the student will usually stop looking for a while. This demonstrates that they can re-holster without looking, but often still look because they don't appreciate the dangers of their actions.
To train yourself or your students to not look at the holster, you must be diligent. Every time you re-holster, make sure you do it without looking. Once you can do this smoothly, start working on adding more stress. Just like adding stress when you are shooting; i.e. moving, walking, taking cover, or engaging a suspect; these same drills should be incorporated into re-holstering. You can begin by being on target with your firearm, then step offline while re-holstering. Once you are comfortable with one step off line, start adding a re-direction with the other hand while moving and re-holstering.
You can expand these drills to anything you can think of just like when shooting. One drill that should be added, probably with a non lethal munitions weapon, is: While stepping off line and re-holstering, you start moving forward to the subject. As you start securing your weapon, a threat appears, the subject reaches for a weapon, and you have to draw again and assess the situation. Any stimulus you can add to these drills will allow you to be better prepared on the street.
Please do not misunderstand the above, if you feel that you need to have your weapon out and on target then keep it out. Your safety should be your first consideration.