Level select:
Successfully complete the game under the normal or hard difficulty setting to 
unlock an option that will allow you to pick any level in the game.

Grenades:
When on any level and there are two enemies talking to each other, throw a 
grenade (for example at the beginning of the Think Like A Gun level). They will not 
even look to see what it was because they will be distracted.

Shooting enemies:
The best way to kill someone is to pull the trigger when your crosshairs are red.

Shooting hidden enemies:
Some enemies hide in areas where you cannot shoot them. Throw a grenade at 
them and they will jump out of hiding. When they jump out, they will land on the 
ground on their stomachs. They will require a few seconds to get back up. Before 
they do, aim and shoot them or they will go back and hide again.