My group has done maiming in the past in a couple ways:
1) As a DM, I actually went and used the combat and tactics guide crit chart, with significant modifications. It allows for all sorts of nasty injuries that last a while. Saves can be made to avoid the effects of the crit (severed limbs, penalties to actions, movement penalties, etc.) by making a Fort or Ref save. It seems obvious to me that the strongest (high fort) characters can withstand a blow that might otherwise take a limb and that the nimblest (high ref) characters can avoid the worst of the damage. Non combatants have bad saves in both these areas and have extra incentive to avoid attacks. DC is set by the base max damage of the crit. If you are a dwarf with 20 strength and a War Axe +3 that you are specialized in, then your save DC's vs. crits will be 10+3+5+2=19. That is the base max damage before the crit multiplier. Thus a War axe is less likely to create limb loss than a Two Handed sword, but does extra damage when it a successful crit is recieved. In all cases damage is calculated as normal, it is merely the threat of the extra crit effects that is different from the base rules.
2) Vorpal weapons go from instant death, to potential to cause limb severing, randomly determined, with head being a possiblility only 10% of the time. Saves are 10+ base max damage for a weapon (that same dwarf with a vorpal War Axe is 10+10+3+5+2=29 DC vs. outright severing). Thus, Vorpal weapons are still very dangerous, but not instantly lethal.
The chart that we have used is (d10)
1-2: Right leg
3-4: Left Leg
5-6: Off hand Arm
7-9: Weapon Arm
10: Head