I'm pretty sure the Sneak Attack rules in D&D 3.x were invented for one simple reason: the players of thieves wanted to be able to Backstab in combat more often. I don't usually play thieves and I'm one of those cranky DMs who think the class is more trouble than it's worth, but I can totally sympathize with the thief fans on this one. A big reason to play a thief is the simple joy of going around shanking fools like it's going out of style.
Somewhere I've seen arguments to the effect that backstab is essentially a non-combat skill. I.e. you can use it if you surprise your foe and not in general melee. My old killer DM (this guy) used to allow a backstab in dungeon fights if you could roll both Move Silently and Hide in Shadows the round before. I never quite figured out why moving silently was important when the half-ogre was bellowing obscenities at the badguys and swords were clanging against armor, but making that Hide in Shadows throw usually got you ignored for a round as well.
I recently stumbled across another interesting implementation while reading the opening chapter of an old Judges Guild module. Michael Mayeau's Survival of the Fittest is one of those choose-your-own-adventure affairs, designed for first or second level OD&D characters. I haven't read the meat of the adventure yet, as I've been kicking around playing this thing solo to see how it turns out. With no DM around to adjudicate lighting, positioning, etc., the special solo rules give thieves a flat 1 in 6 chance of being able to pull off a backstab at any given time.
That seems like a good rule of thumb to me. In any given round where a backstab seems possible, throw the die to see if the thief can make an attack from behind. If the roll doesn't go the thief's way, they can then decide if they want to Hide in Shadows to set themselves up for backstabbing on the following round.
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