I’m just a lowly DM but I would say from my experience that you must display absolute confidence. It’s imperative that these people trust you. The more they trust you, the better they will pay attention and retain what you teach them. Remember, it goes well beyond the time you have them for certification. I have been in situations years later and remembered clearly what an instructor told me and been successful and had fun. I am an active duty Infantry Marine and sometimes on the weekend I don’t feel like shaving. Then I hear the voice in my head of Drill Instructor SSgt Phillip Jordan saying "every Marine will shave every day". Guess what I do next? Shave. But I digress.
Also, I would recommend that you maintain situational awareness as to the pace of instruction. I have had instructors that belabor a point and you can clearly see the students switching off. If you are teaching younger people it’s important to remember they are very adept at multi-tasking and hence don’t always have the attention span you’d like. Conversely, be careful not to go too fast and realize that some people aren’t comfortable raising their hand with a question in the presence of their peers. This can easily be overcome by again earning trust and making sure everyone knows you are available off line after class or on the phone.
When teaching to this interactive generation I find the Socratic method of instruction is very effective. If you’re not intimately familiar with this, I would recommend you get that way. If you need help just let me know.
Lastly, be passionate, positive, and motivated and you will find it is contagious and pays huge dividends. Know that you can’t reach them all and that some people just aren’t cut out for this sport no matter how much they wanted to be. The art behind the science as an instructor is to tell who they are. Luckily, they are not many in number it appears. Have fun, good luck, and safe diving!