Your 8 digit security code is normally the date of birth of the person who set up the account for example 14th February 2012 would be 14022012. It should also be the answer to your security question so if you can go online it should ask your security question and hopefully that will jog your memory.
I have also heard that in some cases the network sets the code so it may be worth a phone call to them before going to a store. Networks are usually happy to help as they want to keep you for as long as possible for obvious reasons.
Good luck, I hope that works!