One reason you may be asked to sign up again is that the cookies we use to recognize members may have been deleted or may have expired. You do not need to go through the entire membership process; you only need to sign in again. To sign in, look for the shadowed box with the title "Already a member?" located on the right-hand side of the membership form. Enter your username and password, click on 'Go' and you will automatically be taken back to the original page you were reading. You now have access to the entire site. For more information on cookies, please click here.
Another reason you may be asked to complete the signup process again is for a contest or SMS but this should only occur once.