In the opening, don't move the same piece twice before you moved every piece once.

I recently play a game where my opponent break every rule in the opening and because of that, he was forced to resign on move 23. All the pieces were still on the board, except one pawn and two minor piece on each side. If you like, see this instructive game. It is very good for beginners to see how important is to follow the rules in the opening.