Free Car Parts
If you want to accrue parts for your car but don't feel like paying for them, "Midnight Club: Los Angeles" has a cheat that allows you to do that. When you are in the "Garage" option, choose "Customize Your Car," then "Performance Parts." Hit X and then instead of choosing A, hit the big X in the middle of your controller that takes you into your Xbox 360 options. Join a multiplayer session. Your car will then have the parts you want without losing any money, and when you leave the multiplayer session they will still be there in the single player game.
Freeway Battle Victories
To initiate a highway race, flash your high beam headlights at a car when you are on the highway. The most difficult opponents will be indicated by a red marker, but with this cheat you should be able to beat them easily. Before the race commences, get behind the other racer and try to work up a lot of speed. When you are ready to start, pass him and flash your lights, which will start the race. By that time you will be well ahead of your opponent before he even starts to race.
Quick Travel
The map of Los Angeles in "Midnight Club: Los Angeles" is quite large and getting across the city can take some time. If you go into the "Race Editor" option, turn off the Drive and Drop mode. Once you do, take your cursor and put it where you want to be in the city. Put Drive and Drop back on, exit the menu and you will magically appear in the part of the city you want to be in! Another option is change to Arcade mode, pick a race near your destination, start the race, quit it and you will then be in your desired location.