Thanks to T-Mobile, my social security, password (which I don't use the same password for anything), checking, and drivers license number.
1) Lock your credit on all three credit bureau (Equifax, Experian, Transunion)
This will stop anyone from applying for credit, because they will not be able apply unless you go and unlock it
2) If you believe your SS# is being used for employement etc, and you have cause, you can have your SS# changed
It's a last resort and a very long process, if you do number 1 you should be fine
Sign up for credit monitoring (LifeLock) is a really good one so if anyone uses your credit, applies for credit, you will know
Use complicated passwords, and don't use the same password on more than one application. Use a password tool like Log Me In, etc that can keep track and protect your passwords.
Nothing is 100% full proof, but these steps will make it much more difficlut.