Go to the "Extras" menu and select the "Cheats" option. Then, enter "PIES" as a code to unlock all tracks and cars in Arcade mode.
On empty race days, create a sprint race and enter it. When it starts, use all your nitro and just keep pressing L near the redline. You will usually win about 95% of the time.
On empty race days, host a Class 1 sprint race. Bet all your money on the Dodge Viper, which will usually always win. If there are two Vipers, do not make a bet.