Most people will experience an excess of personal belongings at some point in their life. Sometimes it seems necessary to keep things around that have purpose, or might have a purpose in the future. My suggestion is to evaluate those items much like a business would evaluate inventory. This is generally done by using a few criteria, with the first being frequency of use. This would simply be how often an item is used, whether is be daily, weekly, yearly, or even less. The second area to review would be the cost to keep the item, which would get a little more specific by calculating the volume of space an item occupies and how much that volume is worth relative to the rent/mortgage cost. The final review criteria is the cost to replace, as in how much would it be to acquire an item again.
We can use a couple of generic examples to demonstrate this concept. The first, we'll say, is an item that isn't used very often, is large (and therefore expensive to store based on cubic feet), and inexpensive to replace. It would seem wise to get rid of it and only acquire it when it is needed again. The second is an item that is useful somewhat infrequently, quite small, and rather expensive to replace. It would make sense to keep it around.
As you might assume, there are many more possible situations, but with this starting point you'll be empowered to deduce the wisest choice for your belongings. Being diligent in working at this will help you establish an effective personal finance policy that will benefit you now and in the future, and in utilizing this principle, used by businesses around the globe, you will have the developed the most cost-efficient allocation of your space possible.