Large Money Off Cars
After you finish rebuilding a car put it in the car lot for a long period of time, then auction it and you will make more money than if you had just auctioned it right when you got through with it.
Do Jobs On Expert Level
On expert level when it says no jobs are available, just go back to the main menu and choose internet features. Choose extras on the bottom, download all seven items and when you come back to play you will do jobs from now on.