Choose a Ridge State Grand Prix that you have not yet cleared. This works especially well with RSGP 12 or 13.
Finish the next-to-last race with a points lead of at least 9. Since you will always earn at least one point per race, there is therefore no chance of finishing the GP lower than 1st.
When the final race starts, do not finish. Wait for the time to run out (3 minutes after the lead car finishes). Proceed past the results and you will find that you are awarded the full amount of FP and CR for winning the GP. However, since you did not finish the final race, you are forced to either Retry or Quit. Choose to Retry as many times as you like; you will continue to earn the full amount every time you do not finish. This will enable you to quickly unlock all of the hidden races and purchase all of the parts...but you will still need to rely on your skills to actually finish the races!