The Firing Pin Journal is fast becoming my favorite gun blog. I shamelessly lifted this pic from the Journal's site, where it is used as an example of proper finger placement on a gun while scanning the scene. Link Here.
Here's the 4 rules:
1. All guns are always loaded.
2. Never point the gun at anything you are not willing to destroy.
3. Keep your finger off the trigger until your sights are on target.
4. Be sure of your target and what is beyond it.