Use passphrases and change out a part of that passphrase with something derived from the site. Of course password managers can be considered safe, but it is still a single point of failure. Passphrases are virtually uncrackable, even if the attacker knows you're using a passphrase, given that one of the words are outside the top 500 words list. If you even throw in a special character somewhere, it'll be impossible to crack. Passphrases makes passwords easy to remember, and given that you have one part of it that's unique to each site, even if that is just a word, you can still remember that one rule very easily. You get rid of that single point of failure, and have just as uncrackable passwords as a password manager does.