Interesting the way everyone has interpreted Shannon's original question. I'm intrigued, though, by Suz's reframing the question as to how might one go about consciously identifying an item as a keeper, even if said item languishes at the back of one's closet for a couple of years--unworn, but not purged.
I've inadvertently done this a few times in the past and, after reflecting a bit, here is my stab at why an item might survive numerous purges over several years and eventually be resurrected to become once again a functioning part of my wardrobe.
1. The item was super flattering in terms of design, color, and fit. This was the big one.
2. The quality of the item was stellar and any signs of age just improved its look instead of diminishing it.
3. I loved the way the item made me feel exactly like me whenever I wore it.
4. The item was functional. It fit my lifestyle and played nicely with other items in my closet.
So that's my criteria for keeping something in my smallish closet, even if fashion has decreed its day is done and said item might not see much daylight for a season or two. Not many items can meet all these criteria so, needless to say, the space they occupy is not that huge--a couple of hangers and a smallish storage box holds everything quite nicely.
Now I'm off to resurrect my 100% cotton, designer, baby boot-cut jeans that I bought in a small shop in London almost a decade ago. The fit was so amazing and the wash was so perfect that I couldn't bear to let them go even when skinnies ruled the roost. I suspect, though, those jeans will be seeing a lot more daylight this fall.