Fill out a card for each site used, ever, in your online work. Blogs, content sites, social media and sharing sites, affiliate sites, individual clients - one card for each site. On the top line write the name of the site - example: Wizzley.com
Below this, enter the email address used, username, password or any other information necessary to use the site. It's easy to replace a card if the old one is filled up with changed passwords or new information. Keep a stack of blank cards in the back of the file for updates or taking notes.
This is much easier than hunting through a notebook of scribbles and unorganized information. The start-up may take a little time, but is easy if you fill out and file a card for each site as you go along. The u[keep only consists of filing the cards and changing to a new card when necessary.
If you have multiple varieties of sites, use color-coded cards for ease of setting up your work schedule. Sites for marketing your work get "green" and sites to write content all get "yellow" or any variation to suit your particular needs.