Infinite Ammo: Collect all of the dog tags and sucessfully complete the game and save.  Start a new game and equip the wig that you have just unlocked.  This will give you infinite ammo.

Press L2 and R2 one hundred times when hanging on a ledge to do pull-ups and increase your grip level up to level Two.

While Snake or Raiden are listening into a Codec conversation press R1 or R2 to hear what they are thinking.