How do people remember all their passwords?

My passwords are divided into two categories -- important (e.g. for a bank account or the main email address) and not important (mostly for a variety of online stores and other places which think that forcing me to register there is a good idea). The important passwords live in a password manager file which itself lives in Dropbox. The unimportant passwords are composed algorithmically, so that when I am looking at a site's login page I can reconstruct what the password for it should be.