If you know that you signed up for FamilySearch with your membership record number, you can remember your birthday, and you have an in with the ward clerk, you can recover your username and reset your password in a matter of minutes.
This is especially useful for youth who have signed up with a parent's e-mail address and have no other way of recovering their username and password in order to do family history work at mutual or for a Sunday lesson.
First, admit that you don't know your username. (Or skip the first two steps if you remember)
Provide your username and "type the letters from the picture" (thus proving that you are a human and not a robot).
Choose to recover your password using your membership record number and birthday and your password will be reset. (Write down the new password just for now)
Use your new password to log in!
Your new password will probably be hard to remember, so change it back to something you can remember by going to your account settings.
Use your reset password to change it to something you can remember next time.