Keep one car for show, and every time you get a new ride forget the visuals and only upgrade performance.
[Longer (more detailed) Version]
Early in the game; You have your first car pimped out, but you now have access to something quicker so you get it. Instead of wasting "bank" on upgrading the "star rating" on your new car just forget about it and only upgrade the performance.
Now with your first car forget about the performance and only work on getting the "star rating" up there, this way you will save lots of money! BUT...
Remember to switch to the appropriate car when necessary; for eg. when you enter a race use the new car, and when you go to a DVD or Magazine photo-shoot use the pimped one as they will give you more cash.
Also every time you unlock new visual parts be sure to get your "show" car out and upgrade it straight away as increasing your "star rating" is required to progress through the game.
This should help you alot until you progress more and start making some serious numbers. ;D
Added by: stevo5196 Dec 22nd 2010, ID#33369