Purchase a car, then sell it at the original price. Refuse the buyer's offer, then offer the same price to them again. Refuse the second offer, then offer a price of $22,001. Before you buy something, repeatedly press [F1]. Then, buy the item you want. You will then receive $1,000 for each time that [F1] was pressed.
Winning cars
The best way to win a car, is to go to the Aqueduct and count the gray square tiles on the side (the same ones that wreck your car). When you count six, ride up the wall to avoid the first barrier, then count four. Ride up the wall and back again. Then, count three and repeat. The finish line is right there, and this is a sure way to win anyone's car. If the numbers are not right take some practice runs on it and count for yourself.
Avoid losses
Save the game every night before you go out. If you lose money, your car or crash, you can easily go back to your old game with everything intact.
