I generally use the "one year" rule. If I haven't read it, used it, or referenced it in over a year - then it can either go in the bin, get sold off, or go to someone else in the family or the local charity shop.
I used to be terrible at hoarding paperbacks that I had no intention of ever reading again - but now just have one shelf and when it reaches the left hand side it goes in "The Bag" which is carted off once it has a sufficient volume of books.
It's harder with things that have sentimental value or hark back to a time of a particular interest or passion - my photography stuff lived on happily for years before the wife finally talked me into "letting go!" ...