Much of this is discussed in the New Mystery Shopper area.
I keep Excel sheets for lots of stuff as part of my overall excel spreadsheet. I keep a list of clients and update who is shopping them when I see it. The list is easily re-alphabetized by Excel for me.
As I hear mention of a new-to-me MSP, I note them on a sheet in my Excel workbook and when I have some time I will take a look at the site, research the company and if it looks reasonable sign up. If I decide NOT to sign up they remain on that original sheet highlighted in red so I don't waste my time researching the same company again. If I terminate an MSP they also get listed on this sheet with a red highlight.
I have a sheet of companies I am signed up with with a link to the login page of their web site, my user ID and my password. I also list the day I registered because not all companies accept you as a shopper when they don't have jobs in your area but may come back a year or so later with acceptance and a job offer. I also list information such as stated pay cycles, telephone numbers, schedulers' names and email addresses etc. as they become available.
All of my scheduling and record keeping are done on monthly pages inside the workbook. A separate page is for legitimate expenses that are not shop specific.
All of this feeds back to a recap page that shows monthly totals, year to date totals, amount unpaid, etc. and then feeds in expenses so that at any point I can see what my tax picture looks like and plan accordingly.
I do operate a separate email account for MSing as well as a separate PayPal account for the purpose.
So in summation, my shopper workbook has a Recap page, and expenses page, a 'services' page of who shops whom, a companies I'm signed up with page, a companies to sign up with page, a page for each month (that includes a calendar for quick reference), a 'shops by MSP' page where I copy each month's information and paste, then sort the whole thing by MSPs and a page where I have just copied in the previous year's 'shops by MSP'. My shopper workbook also includes pages for other tax deductible items such as medical appointment w/ copay made and mileage, prescription copays etc., it includes an income & expenses page for my livestock business and a page for charitable deductions and mileage. In other words, the workbook is an accumulator of information I will need for taxes. It gets backed up frequently to my desktop computer though I am working on it on my laptop or desktop from a thumb drive.
If you have a modicum of experience with Excel it is not horrendously difficult to put together a workbook for yourself that meets your needs and repair/change it if/when something goes bonkers or you discover there is another piece of data you want to track.