Enter "735S653J" or "585A926D" as a code under the "Excusive Content" option at the main menu. If you entered the code correctly, a message will appear. This will unlock extra money for equipment in Single Player mode, extra money for character unlocking in Multiplayer mode, silver weapons in Multiplayer mode, and the Superb Ranger weapon that can be used during the first two chapters.
Successfully complete the game to unlock the Very Hard difficulty.
Jumping and switching weapons while in mid-air is a good way to help keep your current speed and momentum. This helps a lot if you find yourself being chased by someone in a multiplayer game and are low or out of ammunition for your current weapon.