Instructions
Buy one of the five Porsche cars and the Chrysler ME-412 and upgrade them. Select "Upgrades," then "Tires." Buy the Perrell tires that will be half off, costing around 17,000 credits. Sell the tires back for almost 21,000 credits. You can do this indefinitely.
Select any upgraded car. This only works if you have not patched the game through Xbox Live. Once it is patched, this will not longer work. Upgrade the car to the second-highest option. Reinstall the stock parts and press "RT" and "X" to sell the parts, but instead of selling your parts, doing this will sell the highest available upgrade parts. You can sell these parts as many times as you want.
Hire a driver with high-endurance racing stats to race endurance races for you. Good endurance drivers will finish high and make you lots of money.