A good way to keep your current speed and not lose momentum is to jump and switch weapons when you are in mid-air. This trick comes in handy when you are being chased by someone in a Multiplayer game and you are low or have no ammunition for the weapon you are using.
At the 'Main' menu select 'Excusive Content' and enter the code 735S653J. Entering this code will give you extra money for equipment in Single-Player. Extra money for characters in Multiplayer and weapons become silver in Multiplayer.