I only cover meaningless games these days (there's a case to say that pretty much all games are meaningless!) but, especially with younger players who might have some sort of future playing career, I make a point of making sure that they know the sort of stupid things that could get them into trouble in higher level games where referee flexibility is more frowned upon.

Totty's point about goalkeepers actively making scuff marks within their six yard box and goal areas to aid with their positioning is definitely one of those I try to ram home. If nothing else, it encourages them to be more subtle, if you get my drift.

I've never been card happy, but like to think active player management (talking to them during the game) goes a long way in keeping tempers down and players remaining on the pitch.