-Knock knock, anyone home? No? I'll just help meself to this Ming vase then.-

When people tell you to 'make yourself at home', they rarely mean it. It's a formality, a matter of courtesy. You still shouldn't lie on their beds since that shirt you've had on for the last half a week has seen much cleaner days.

Always drop in on the parents of the host and drop them a hi. It's only polite to do so.
You absolutely must not congratulate the lady of the house on 'being with child again', regardless of how classy you might think it sounds, or how confident you are that she is indeed pregnant, despute her rather old countenance.
Don't go snooping in people's rooms if not escorted by the host(s).
Having participated in an aggressive game of volleyball/badminton/popcorn-throwing, refrain from taking your shirt off and lying prostrate on their nicely waxed marble floors and attempting to make sweat angels.
Whatever you do, you're NOT to drool on their pillows, bolsters or favourite stuffed horses.

And just something I randomly picked up from a friend that really has no business being here: Wandering eyes that do not belong should keep theirs to themselves. And their shrivelled lips for that matter.