I promise myself that I will do this every year when I pack down the Christmas decorations, and so far I never have. So come late November when I am trying to work out what I need to buy/replace/make I have to pull everything out and sift through the boxes.
Make yourself a simple list (you can even do a paper one if you wish). Mine is broken down by category – e.g. decorations by colour, number, material (fabric, glass etc.), shape and size, placemats by colour, shape and number. Believe me it is useful when you are trying to work out if you have place settings for 15 people