If at all possible, keep pets out of the house. Music is mellow. But not when showing a house. Turn off the radio or television. Let the salesperson and the buyer talk, free form disturbances.
Silence is golden.
Be courteous, but don't force conversation. Try to stay in the background, but be prepared to answer questions if asked. The potential buyer wants to inspect your home, not pay a social visit. Never apologize for the condition of your home. After all, it has been lived in.