Fix parts
In order to fix any and all repairable parts while they are still on the car without the need to take them off, press [Delete] + [End] + [Page Down].

Free job
When you are in job mode, press [Insert] + [Home] + [Page Up]. The job will finish by itself, you will get to keep your money and you will get paid.

Free parts
Just take parts off people's cars then use the "Free job" code. You will still have the parts under that car name and still be paid the full price.

Hints & Tips

God-like mode
Preform every job you can. After it says that there are no more jobs left, obtain a cheap car from the auction, fix it up, and attempt to get more than $20,000, which will let you reach God-like mode.

Easy money
Note: This will not work on any high priced or third party cars. Purchase an old or new junker from the auction. Do not fix this car. Then take it and buy a new car door from the catalog. Place the door or whatever part on the car and auction it off. This will make anywhere from $78 to $1,000 profit.