Very simple! I give them a reason to sign the guest log. I usually set up a small table with light refreshments and the guest log. When they arrive, I shake hands and say, "Hello, I'm (agent name) with (Realty Firm Name)! Welcome to your future home! Please help yourself to light refreshments and please sign in my guest log. Take a look around and if you have any questions, please feel free to ask!" If they go for a cookie, they will generally sign in.