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 multi-player game and are low or out of ammunition for your current weapon.
Various bonuses
Enter "735S653J" 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 a bonus weapon that can be used during the first two chapters.