I carry, on a lanyard around my neck, a 128GB flash drive, that contains an Excel spreadsheet, with all the accounts I have to log in, user names, and passwords. Currently over 300. The Excel file is password protected, as well as the flash drive. I only HAVE to remember 2 passwords.
It is impossible for any person to remember all the different usernames and passwords needed, to do a business like ours.
I also have, encrypted backup files for all my tax programs, and other important documents, on the flash drive as well. If I leave the house, it goes around my neck. I have a complete backup of all data files on a separate external drive at home as well.