House Rule:

If a character targets a specific body part on an opponent, I will add a size bonus to the target’s AC (hitting someone in the hand is more difficult than just hitting them, hitting their eye is even more of a challenge) Depending on the part to be hit, the creature’s body may also provide cover.

A successful hit against the body part will cause a -2 penalty against any relevant skill checks or saves (eyes = spot, search, read magic, etc, hands = use rope, climb, attack etc.) If you want to completely sever a body part, I may assign the part a certain number of hitpoints. This may be more or less than the creature as a whole because it may be more difficult to chop off someone’s leg than to kill them by running them through.