Good advice so far. A few more ideas:
Realize that different students learn differently - be ready to modify your delivery as needed.
Some instructors get too enthusiastic to help their students to learn everything the instructors know about flying. If you're presenting to a bunch of PPL students, you can use a test prep to help you figure out how much time to devote to different subjects, and which subjects can be covered lightly or skipped. The FAA creates more test questions in the subjects that they consider to be more important. But remember, too, that some subjects are simply more difficult than others. Ask lots of questions as you cover the material.
Most students underestimate the amount of studying necessary to master the material. Encourage them to work problems at home, and provide them a schedule so the can read up on the material before class.
Some of the subjects students get most challenged by: altimetry, weight and balance, take off and landing distance graphs, pressure altitude conversion, regulations, VOR Nav, E6B calculations, segmented circle/traffic pattern orientation. There have to be others...
Try to start each class with a quick review of some of previous class, to tie it all together.
Keep it light, and fun.