Contact someone you know and trust with a bank account before you write anything on the back of your check. You may also want to inquire tactfully about his ability to cash the check for you. Many banks require the third party to have at least the amount of the check in his account before a sign-over is allowed.
Write "Pay to the order of" on the back of the check first. This is also known as a restrictive endorsement. This should be at the top of the back, where you would normally sign your name first.
Write the person's name underneath "Pay to the order of." Some financial institutions don't require this step, but it doesn't hurt to do add clarification. You won't be penalized for doing so. Just make sure that you still have ample room for your signature underneath.
Sign your name to make the transaction final. Your signature should go just underneath the third party's name. It also helps your cause to be present with her at the bank teller window. It's not always required, but with both parties there and identification in hand, it's less likely that you will get snagged in payment-delaying red tape.