Most everything has already been said here, but it is crazy to enforce password changes... One of my clients is a financial advisor/Brokerage. the clear through a larger firm that gives them access to account info, trading, wire transfers etc thorugh an web based interface. Clearly it is critical that access to this is secured. However since passwords are changed every 90 days the employes have lists on their desks with previous passwords crossed off and current ones at the bottom. I or a cleaning person could easily gain access to their customers information and even their assets. I have repeatedly pointed out this vulnerabilty to both the users (so they stop writing them down) and to the IT people at the clearing firm.. but there seems to be a mental block at these large corporate IT departments and they insist on having the password changed and they cant even use some large number of previous passwords.
I personally recomend people have several, I have 6 (not including slight modifications to meet password standards for a site) password of various security needs. a couple highly secure passwords (easy for me to remember but very hard to guess) for things like online banking that you tell No One. Somewhat secure for things that you would like to be private but are not super critical that you may share with a wife or good friend for things like a social site etc. and something simple for the miriad of sites that ask you to create an account and you are not sure how safe a password is in their hands and that I could care less if someone breaks into. Now if my simple password is "dogname" I may have to modify it to dogname1, Dogname, or Dogname1 but when I visit that site I havent logged into in 6 months I generally know my password. But if I am forced to change my password periodically I keep a document titled with the site on my computer (which I can access remotely), but it bothers me when I know that I keep my important passwords secure and feel I am forced to be slightly less secure by having the password recorded in a file on my computer.
I never have to write down my passwords as I know my 6 passwords and what sites I would use them on.