Have your DJ announce / remind people to sign the guest book at some point during the night. Ours got found and signed, but only by about half the guests. Sure, some might have not signed even if an announcement was made, but for people who didn't realize it was there or who were planning to “get to it later” it would have been a good reminder. I also thought that maybe a cute sign on the exit door asking people if they signed the guestbook might have helped out.
This raises the question: what are y'all using for guestbooks anyway? And have you checked out our archive of offbeat guestbook ideas?