If you want a golden rule that will fit everybody, this is it. Have nothing in your houses that you do not know to be useful or believe to be beautiful.
Useful and beautiful (in my eyes).
I have always loved the Pre-Raphaelites. If I had studied history of art it would have been a one horse race. But it is not just their artwork which I adore. It is the whole ethos of the Arts and Craft movement; the belief that a healthy society was one which respected craft production and did not rely entirely on division of labour and machinery for the creation of products. The use of natural and local materials in production. And of course William Morris’ famous quote above.
Beautiful
I know that there is much in our house that does not meet that criterion. From where I sit to work I can see a carving of a fish. It is pretty, but I don’t love it and I don’t know in my heart it is beautiful. There is an incense stick holder which has been superceded by a more beautiful (actually more simple in design) and more useful one. In my pen holder there are two marker pens which I know don’t work. Outside in the courtyard I can see an old bird table that has been superceded by are more practical one. There is a glass contraption that was supposed to trap wasps but never did. All that and I didn’t even leave my seat.
Useful
Join me today as we walk around our homes. Look carefully at the things around you and ask yourself if they would pass Morris’ test. If they do not, then why are you keeping them?
Neither useful nor beautiful
one pair of shoes at a time 
This is one of my favorite measuring sticks for when I am trying to decide if something stays or goes. Its a good one.