To prove the standard mileage deduction you must keep a written diary of where you go, who you see, why you went there, and what purpose you had.

For the actual cost method you must keep all receipts in addition to keeping a mileage diary.

You can claim the actual cost method so long as you never claimed the standard mileage method for that car.

You can go from the actual cost method to the standard mileage method at any time.

Once you use the standard mileage rate method you can never go back to the actual cost method.