All levels and all skins
At the start quickly press Up, Down, Left, Left, Right and you should hear a silly sound.
Bare Person
After you kill the leader of the Irish mobsters in the mission "A Midsummer Night's Massacre", Predator is hanging the leader by his feet. The cool thing about this is the leader doesn't have skin anymore. Predator ripped it off. Predator calls it a "Trophy!!" Its cool!
Combos
There are three types of combos. The first is the combo that always ends with a strong attack. It can be executed by pressing the attack buttons in random order, for example Triangle(2), Circle, Triangle, Circle. The second type of combo is the one that ends with a heavy attack. It is executed by pressing Triangle, Circle, Triangle, Circle, Triangle. The last type is the combo that ends with a light attack, usually an impale, that is followed by tossing the opponent in the air. It is executed by pressing Circle, Triangle, Circle, Triangle, Circle. Combos always contain five strikes, and they end with a heavy attack except for the light combo Circle(5).