I keep a worksheet germane to the particular shop for each and every shop. On it are all of the pertinent details of that shop (date, etc). Stapled to it are the receipts, business cards or whatever was required by the MSC to obtain in doing the shop.
The worksheet is files into a folder of the month in whit the shop was performed. That means, all shops whether they are gas stations, banks, retail, etc all go into the folder for that month.
When I receive my pay detail for the shop of that month, I make a copy of it and add that to the folder. Often, the pay detail covers multiple months. I make copies of it for each month's folder of the occurrence. I hold onto it until I've resolved each shop's payment, checking off the payment received. Once resolved, I file the folder away into my file cabinet.
For mileage, I keep a yearly date book with a month by month mileage amount written down for each day that I performed a shop. As an example, for March 17, I might write down the name of the furthest city I drove to, then the number of shops I did for that day and lastly, my mileage total for the shops done.
I might also add that I write down on January 1st of each new year, I write down my odometer reading. I also write down the odometer reading at the end of the year too (should be the same).