Reading recently, I came across this and it resonated with me:
"To Do lists are not lists of things to do. They are lists of things you can do." (from "The Blank Screen" by William Gallagher)
Too often I have put things on my 'to do' list that I can't actually do because I am waiting on someone, do not have all the information, don't have the tools to do it, etc. These 'can't dos' clog up my day and carry over, making my list get depressingly longer and longer.
I now get rid of them as I am never going to able to do them. I shunt them to a 'someday' file which I periodically review. They could become a 'can do', but more often than not they simply disappear.