Spreadsheets (Excel, OpenOffice Calc) are the best way to manually manage your accounts. I use databases (MongoDB) but I also write my own software so when an account is created it goes right into the database instead of a file, with its profile details, proxy config, and credentials. Same database also tracks the accounts activity for me to review if I need to.
A database is also handy for cross referencing how many accounts are tied to a proxy, or how many different services are tied to an email address, and so on.