If you're not expecting to be specifically targeted, then "modify a single password per service" can be surprisingly secure. Don't just add "tw" "fb", but memorize a more complicated algorithm that's not obvious from inspecting two or three leaked passwords. e.g. Basic Caesar Cipher on the odd characters of the passwords using some part of the service name (fb, tw) as a key. Memorize a single algorithm that you can do mentally. Use something completely different for primary accounts (probably bank + main email that allows you to reset other accounts' passwords).
Some people will disagree and say "just use a secure password manager", but there is a valid argument that managers are not necessarily the best solution, depending on your use case.