Having gone through this with my wifes car a couple of months ago - all i can do is advise you to be prepared.
Get all your servicing records, receipts, V5, MOT docs, pictures showing its excellent condition (Progress thread documentation) in order; and starting looking for similar spec'd cars to yours with high prices and print them out etc
Its someones job out there to try to get away with paying you as little as possible !
Be one step ahead...........