Just my opinion but I like to use one good password for each site I use and keep it. My passwords are so long I can't even memorize them. If you are really paranoid your passwords should be 10 to 15 characters. Random upper case, lower case, and special characters.
I think its best not to use words... any words. If you're set on changing using a set schedule I hope someone has some good advice. I'd be interested in seeing the reasoning behind that approach.
If you're concerned about spam, use disposable email accounts when you register for new sites.