I have a few fairly complicated passwords for stuff like shells, emails, and bank accounts. For other services and throw away accounts, I usually have a constant word I use (i.e. if the login is for a game, it'd be 'videogame') and that follows by a hash i calculate of the service's name in my head. So you can do something like add every other letter's value (a=1,b=2,etc) or number of characters in login url divided by value of last letter.

This "hash" is fairly simple for me to calculate, frees up my memory and will not overtly compromise all throw-away accounts if one of them leaks.