From Miss Manners:
“Nosy people have already proven themselves to be rude, so you should hardly expect them to make tactful remarks. The important thing is to cut them off at the first question. The only explanation necessary is, “That’s personal.”
“But you must also … not [to] fall for two common arguments: that curiosity is natural and that people who don’t disclose personal information must be ashamed of it. Dignified people value their privacy, and being curious is no excuse for demanding that it be satisfied. Under such pressure, they should merely smile and repeat “That’s personal” as often as necessary.”
The complete context is here.
Curls n Skirls 