A Google+ discussion led me to hammer this out and add it to my rules doc. I use Saving Throws to help adjudicate this sort of thing, as it's a good gauge of combat ability for both the attacker and the defender.
Trip: A Fighter, Paladin, Ranger, Cleric or Druid may forgo his normal attack and attempt to trip his opponent, knocking him prone. The Attacker must make a Saving Throw, to set-up his opponent for the maneuver. Then, he must score a To Hit, which will do 1d4 points of potential Damage (no other modifiers apply.) The defender must then make a save, at a penalty equal to the Damage scored. If successful, the Defender remains standing and takes none of the Damage. Otherwise, he suffers the Damage and is knocked Prone, suffering all of the effects of that condition.
One question that occured to me on the above, was whether or not the defender's armor would come into play. I decided, yes. I can see where armor would, at least sometimes, aid the defender in fighting off a Trip. Especially, when magical armor or items, are in play. It's fiddly enough, without breaking it down further.