Use fraud alerts. You can call any of the credit bureaus and put a fraud alert on your account. Once you call on a credit bureau, it will notify the other two and the fraud alert will be active at those bureaus as well. The fraud alert warns a potential creditor to do additional due diligence before extending credit. The idea is that credit reports with a fraud alert have already been compromised, so the creditor should do extra work to ensure that they are giving credit to the right person. The bureau isn't required to do this, but they probably will because ultimately, they could lose money.
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Credit score monitoring. Some say credit score monitoring isn't necessary, certainly not one with a monthly fee, but you can use free credit score monitoring from Credit Karma as an early detection sensor for fraud. Credit Karma doesn't give you a the official FICO score —it's a TransUnion credit score using TransUnion data—but if you notice unexplained changes in your score, it might be a sign that someone is stealing your credit. With these techniques, you can do what many of the monthly identity theft monitoring and protection providers do, without the monthly fees. This also has the added benefit of reducing your junk mail, which is a boon to you and the environment, and helping you sleep better at night knowing your information isn't floating around for someone to steal.