Well, I don’t know about clients, I just know how I handle the 15-20 personal accounts I have across various social media sites. I actually don’t trust apps much even though I know that there are plenty of them people like to use — LastPass comes to mind. For my own passwords, though, I like to go old school and use an actual physical notepad. I have one page for all of my emails and another for various social media with my username, the email it’s registered to, and the password. Obviously, I also try to change the passwords every few months as well. In your case, you could have a page or even a spread dedicated to a single client. Even better, have whole folders dedicated to a single client. At least paper isn’t hackable, you know? Or if you’re not a technophobe, get LastPass or keep all of the data in an excel sheet.