1. Use a password pattern. Something like 8 random digits that you memorize and then part of the domain name or business name. Such as "goo" for google. Put them in whatever order you like. Now you've memorized one pattern but use a unique password everywhere.
2. Use a predictable algorithm instead of a password. Their are web based services for this. You enter the domain name and then "encode" it to a password. That is typically not reversible.
These fall down some when you have to change a password or when a system has requirements that don't match your password (like requiring a number or symbol). Other users will mention other limitations as well.