I prefer the hit location rules from the Top Secret S.I. rpg. In short, all you have to do is roll 1d10 and consult the table below:
0 = Head
1 = Right chest
2 = Left Chest
3 = Abdomen
4 = Right Arm
5 = Left Arm
6 = Right Hand
7 = Left Hand
8 = Right Leg
9 = Left Leg
Anything else pretty much is left up to common sense. Such as if you want to see if the shot is in the back, that's pretty much depended on the angle of the shot, such as if some one's running in front of you in a chase. The person who's behind will pretty much shoot the person running in front in the back.
If you wanted to, you could expand this table to use a d12 as follows:
1 = Right chest
2 = Left Chest
3 = Abdomen
4 = Right Arm
5 = Left Arm
6 = Right Hand
7 = Left Hand
8 = Right Leg
9 = Left Leg
10 = Head
11 = Left Foot
12 = Right Foot
Really easy hit location rules.