RULE 1: No autohitting. Basically a player may type that they're shooting at someone, but they're not allowed to type that they hit them. Exceptions to this are multi-hit weak attacks that could turn out to do nothing, but of course the effect of the attack is left up to the attacked.
RULE 2: No godmodding. If a player is shot by a gigantic fireball they won't shrug that off with no damage. This also ties in with autohitting meaning no 1 HIT KO's.

I would say those 2 rules are the most important ones in pvp. I would also say that the players fighting each other should at least talk it out a bit in PM if they want to.