Never carry your social security card -- or any documents or I.D. cards with your social security number on them -- with you. We can't repeat that enough. That also goes for your birth certificate and passport. Keep these documents at home, and keep copies of them in case they do go missing or are mishandled. A stolen social security number (SSN) can be used to wreak havoc in all sorts of ways, including allowing access to bank accounts, credit accounts, loans, driving records, medical records, employment information and tax information. And that's really just a list of the biggies. The thief also gains the ability to open new fraudulent accounts.
There are more never-dos when it comes to your SSN. Never give out your SSN over the phone or send it through an electronic form on an unprotected computer. And never use your SSN for identification; for example, never write it on your checks (and don't have it printed on them when you order new checks). Avoid using it as an identifier on such things as your driver's license and medical files -- request to use an alternate number.