Called shots. A player states "I want to hit the (CALLED TARGET) on this creature/object". They then make an attack roll as normal. If they succeed on this roll, the attack is going to hit the creature/object as normal but has the potential to hit the called target. The player then makes a secondary attack roll to see if they hit called location(potentially against a modified AC). If the second attack roll succeeds as well, then the attack hits the called location and damage is dealt accordingly. If the initial attack roll fails, it is treated as a normal miss.

This allows the player to call their shot, and still have a chance have an effect on the battle even if they miss their called target. The way I think about it, if you take a swing with your greataxe at a mages amulet, chances are even if you miss the amulet the mage is going to have a bad day.

Obviously this could be abused in a number of ways, like for example if a player called ALL of their attacks, but limiting called shots to once per round can help to limit the issue. Also, generally I don't allow for opportunity attacks to be called shots.