In career mode, go to your garage and do something with one of your cars (for example, switch a part or get a different paint job). The game should autosave. Then, go back to your race menu. Go in a sprint and bet all your money on one car that looks the fastest. It it does not win at least three races out of four, turn off the Xbox and turn it back on. It will load your last autosave. Try the same sprint again until you win then find a different sprint and start the entire process again.