I heard a story once:- a woman was told that she could buy anything she wanted in London's Bond Street, provided that she would keep it forever. She walked up and down for ages, looking in shop windows at the magnificent jewellery, the wonderful designer clothes, the handbags, the shoes, the expensive pictures and ornaments ... The day wore on, and she couldn't decide what one thing to buy. As it began to get dark, and the shops started to close, she bought an apple from a street trader and ate it.
I've tried this test when tempted to buy yet another candle holder/pretty, old-fashioned salad bowl or somesuch from the charity shop/the latest kitchen gadget, etc.
It works probably ... um ... 50% of the time ...?
'Do I like this so much that I will want to keep it forever?'
(Also, a useful mantra when clearing out wardrobes!)