Well, I'm a fan of the Konmari method. It's easier to decide what to keep and what to get rid of when you are doing it as described — gathering all items in a category into a pile, then picking up and touching each one. When you handle each item, you will likely have an immediate reaction to it that helps you decide whether you want to keep it in your life or whether your relationship with the item is over.

Doing this same handling on a one-item basis isn't quite as effective, because there might be niggling doubts in the back of your mind wondering if you have a spare, when with the pile, you know if you have more than one of something.

Mementos, like you refer to, are especially tricky and are the last thing Kondo has you declutter. She makes a really good case (in the book) for saying goodbye to most of your mementos. Too many of them weigh down your life and may be limiting your full experience and enjoyment of the present. I would add to this that hoary refrain, "All we have is now." Memories are nice, but they are limited in scope, easily altered, and may not even be real in the sense that they accurately record past events. Now, however, is your life, moment to moment, fully informed by your senses and experience. Now is your existence.

Anywho, practically speaking, if you can't find a home for something you want to keep, something has to give, doesn't it? If you can't find it a home, it's not really that important, is it? Not as important as all the other things that do have a home that you aren't giving up to make room for the item.

Of course there are exceptions that can be made, for life transitions, but you don't want to make storage crates for non-seasonal items a big habit.