Actually a dirty (unswept) track/lot will help you learn better throttle control because if you apply too much throttle.. you'll spin out. So finessing the throttle will help stablize the car. Of course if the lot is too dirty.. it might end up being a waste of time and also tear up the car. It's kinda like racing offroad I guess.
Best bet besides getting to the track early to practice is find the cleanest lot that most resembles the surface that you will be racing on and practicing on that. If you can.. get a leaf blower. I'd run a soft suspension setup with no sway bars. Rubber tires of course. TakeOff tires work pretty good on parking lot tracks.
Don't forget, the "practice" dirty lot car setup will more than likely not work on the actual racetrack. So be sure to write down both setups and change them accordingly. Also remember.. some practice is better than no practice. So just go out and enjoy the hobby!