Someone once told me:
Miss Firstname or just Firstname for a girl under 18
Miss Lastname for an unmarried woman
Mrs. Hislastname for a married woman who had taken her husband's name (even if subsequently widowed or divorced)
Ms. Herlastname for a married woman who had kept her name, or a divorced woman who'd gone back to her maiden name
I disagree. To me, Ms. is a perfectly valid honorific for a woman, married or unmarried... it's "marriage-neutral." I did change my name when I got married, and I go by Ms. Hislastname. I don't mind too much if introduced as Mrs. Hislastname, but I prefer Ms.
When addressing other women, I usually go with Ms. unless she asks otherwise.
It did annoy me when someone introduced me to his 5yo son by my first name.
