Do be welcoming. Greet your viewers with a smile but don't try and bond with them. They are there to look at your home not become your new best friend.
Don't look surprised when your viewers arrive. They will think that you have forgotten that they were coming and that'll just embarass them and make them wonder if you are serious about selling your house. If you have forgotten about your viewers, shame on you.
Do have the house prepared for viewing. Windows and glass gleaming, table lamps on, toys tidied away, bathrooms and kitchen fresh and sparkling.
Don't have dirty dishes lying about, unmade beds and washing drying over radiators. Some viewers may not be able to mentally remove the underpants from the radiators. They will not see potential, only mess.
Do decide which member of the family will be showing the house.
Don't do what my husband and I did. Me doing the showing, my husband bringing up the rear affirming my every word like a parrot. Me; "This is the master bedroom". Husband; "Yes, this is the master bedroom". That just sounds weird.