You all have good information to share. I have been doing Mystery Shops for 12 years.
First I make a list of the companies that I shop for, and there police on retaining paper work (copies of shops). I have found they all have different policies. From one month to 6 months, I have a folder for each company and one for each month (exp. ABC company wants it held for 3 months I set up 4 folders, one for current, one each for the next 3 months, at the end of the forth month I trash everything in that folder into the paper slicing. The bond paper goes in a bag for recycle) that I have to hold paperwork for,
My husband and son also perform shops; we place a large white board in the hall by the office.
We have it split in sections by companies and list each company shops under proper section; with W, F, D, for who is doing the shop, Due Date, with a color codes: Red X = need paper work, Green X = have paper work, Orange = Must Brief online. We have a file holder with a holder with everything clamped together for the shop. If it is a merchandising or POPshop, we have a note on the folder on what we need to take with us for that day shops. We also have a large calendar on the door marked for who has shops on which days.
I also use Excel for keeping records of all shops with a different book for each company.
It is important to keep pay stubs also, as if your income is less then $600. The company is not required to supply you with a 1099-M.
Be sure save these records on disk, hard copy, or whatever way you save work off the computer. JUST IN CASE OF A COMPUTER CRASH and you lose data information. It happens! I have two drives on my computer, and save records on both drives in case of main drive crash.