Thankfully systems like 1Password have made this process automatic and still secure, across multiple devices. You can even put all your passwords on your iPhone :-)

That said, for things like PIN numbers for credit cards, etc, you can come up with some reasonably secure but still not easily guessed systems, such as using the last digit of each quad of digits on the card or two pre-decided groups of two. Different PINs everywhere, hard for anyone else to guess, and not hard for you to figure out :-)