the minimum you must have is the username, if not how ? !!!!!
of course is better to store the usermane and the password before all.
for see your password is easy, use
help - Login / password - I've lost my password
see this information and store on one usb key.
the idéa of @daniel1111 is very good for get back your information if you have lost it, but the condition is you have use one site for buy.
then i can only give this advice, store your information now. ;-)