You will need to have an XBox Live Gold account.
1. Sell an item from your Storefront. It can be a design, tune, or vinyl group. Vinyl groups are easiest. Make something simple, have a friend buy it. Alternatively, you can create a second XBox Live account and get one month of Gold for free and use that account to purchase something from your main account.
2. You will receive an in-game notification in your message box. This can take a few minutes. Do not delete this message. If you press "A" you will go to the Storefront. Get the idea? Good. Let's begin.
3. Buy a 1969 Chevrolet Camaro Z28. Go to the Upgrade Shop and then to Conversion and then to Engine Swap. Purchase the first engine swap, a 2010 Camaro V8. Now go and purchase all the the best upgrades in the Engine and Power category. Repeat this for all 4 engine swaps.
4. Exit back to the main title screen. This is the top level screen. You can hit "B" until you are there. Now press "A" and enter the game. It will save the game, and then head back to Engine Swap.
5. Sell all 4 engine swaps. Press "X" to sell a part. Now hit your left trigger button. This will take you directly to your message box and the previous message you received notifying you of a transaction in your Storefront. Press "A" to go directly to your Storefront.
6. Now at the Storefront, press "B" to go all the way back out to the main title screen. Once there, press "A" again to load and save the game. You have now made about 300,000 credits. Go back to the Engine Swap for your Camaro and you will see the engines are all still there.
7. Repeat this as often as you like. You may find if you do this process too fast, your message box may be "empty." If this happens, try going back to the dashboard and reloading the game. Your last transaction will not be saved, but this is no big deal. Load up the game again, and see if the prompt to go to your message box comes back. If not, have someone purchase another item from your Storefront to get the message back.
8. You can build money quickly using this trick. I advise you to invest it in cars ASAP. Turn 10 will likely be patching this soon, and in Forza 2, some patches also reduced a player's money to a set cap. The Ferrari and Shelby dealerships have the most expensive cars in the game.