Don't panic. Just be pleasant but not too overpowering. I was once trapped in a house with a woman who told us her entire life history. she was stood between us and the door. Nightmare.
Tips Put on all your lights. Let them go into rooms by themselves. It will seem cramped if everyone is in the bathroom. Do upstairs first, so you don't tramp mud up your stairs. Turn off the tv. Sounds obvious, but don't point out faults/problems. If they ask why you are selling, don't tell them the house is too small. Make it up if you have to, or a general 'time to move on' Good luck! Oh, and offer them the change to have a wander around by themselves.