You may probably have heard of magical number 7. http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two
I use it as my direction to clear out stuff. I believe that processing possessions is like processing information. If there are too many possessions, I can't remember which items I've got. Although sometimes it is not easy to apply this rule on collections, I try to divide the collections into 7 or +- 2 categories and each category I usually collect 2-3 items, or at most 7 items.
I use a replacement method for collections. Buy one and get rid of one if the buying one is better than the current collections, so that to make the collections better and reach the "most valuable" collections to me.
I do agree collections may not be stuff, but if there are too many, I would still feel a certain stress on it. Feel free to leave any comment^^
