I'm stunned at how much private data the average citizen will divulge. I was buying some stuff yesterday, and the clerk at the checkout asked the customer in front of me for her phone #, which she was quite happy to give. Next, I was signing up for gym membership, and the guy in front of me wrote his social security number on the membership form! Why on earth does a gym need your sosh?

So here's what I do.

Check-out clerk: "Can I please have your phone number?"
Me: "It's unlisted"

I have never had a single issue with this line, and it's not as argumentative as, "no, drop dead."

As for SSN.

Gym Membership guy: "You didn't complete the form, I need your SSN."
Me: "How will my SSN be used?"
Gym Membership guy: "I don't know."
Me: "I bet it's not needed."
Gym Membership guy: "You're probably right"
Me: "Ok, let's leave it blank for the time being."

End of story. Again I'm being civil, and leaving it open ended, even though I have no intention of supplying my National Identity Number, oops, I mean social security number.

In the rare case where someone thinks they REALLY need my sosh and I don't think they do, I'll enter my SSN but flip a bunch of numbers around, I take advantage of the fact there's no checksum digit in a SSN! :)