I keep a list of items that I really want to add to my wardrobe and I give myself permission to purchase them when I find them. These items are (usually) classic items that will work in many seasons and with many outfits. This list is usually quite short and consists of items that have been hard for me to find.
Do you have a "unicorn" item that you have been searching for? What is your rule when deciding what to purchase for your wardrobe? Do you have rules? I would love to hear all about it!