then you aren't trying hard enough. until i put the harness in the car, i was always sore after autocross. i even had that device that holds your seatbelt tight... didnt work well enough.
i cannot stress enough how important seats and a harness are to even just an amateur autocrosser. i LOVE my seats for daily driving. for autocross, being held in the seat tightly allows you to only have to utilize your arms and legs for control allowing for faster, smoother times. it also does well to improve your focus. with your body flailing about, you have to fight the forces AND the course / speed of the car.