The first and simplest one is Fill The Frame! It is the simplest thing for the untrained to do because it doesn't need much explanation and it has clear benefits. Then you move onto cropping, thirds, depth-of-field etc. Only after is grasped can you then discuss when to break the rules. :-)

And another clue about portraits is to not have people stand close against a background. *That* makes it look like a mugshot. They need to stand away from the wall or whatever by at least 3 feet. 5 or 6 is probably best.

Wade.