How do you deal with people who ask prying questions of a personal nature?

I usually respond with something like "That's kind of personal don't you think?" Or if the person is a repeat offender I will say "Oh my, that is a rude thing to ask someone! It's really none of your business!" Or "Why would you ask me such a personal question? Where are your manners?" Don't sound sarcastic or mean when you say these things. Just sound calm, sincere, and matter-of-fact.