Someone linked to this from another forum and I liked it so much I wanted to share with you:
http://www.ucsc.edu/currents/01-02/04-15/rules.html
This is the bit I particularly liked, referring to the "rule" about always using the nominative after the verb "to be":
My advice would be this: If someone knocks at your door, and you say "Who's there?" and what you hear in response is "It is I," don't let them in. It's no one you want to know.