How to keep it private? Don't give the information at all (quite impossible these days)
Don't join social networks - eg. don't join fb, twitter, you get the drift.
Trouble is, if your friend takes a photo of you at a birthday party and post it on his/her facebook and tags you, you can be found.
When a friend posts on her/his twiter "Happy Birthday terrorb2" your date of birth is exposed. So how?
As long as you key in the data online or someone keys in the data about you online, it can be found.
Even emails are not safe. Identity theft is easier with so much information online. So be very careful of what information you give out.
Those "FREE" contest which require your emails and phone numbers - you need to check out the organizer or they too may be a scam to get your personal information. There are even website that say "check the strength of your passwords here" could also be just a scam to collect more passwords for their dictionary attacks.