It's not what you think. By visualization I mean have a clear vision of what you want to accomplish BEFORE you even start doing any work. It's not my idea - I got it from Peter Walsh's book, It's All Too Much: An Easy Plan for Living a Richer Life with Less Stuff.Peter is so right on when he points out that before you even start to tackle the clutter/mess, you've got to have a vision for that particular space/room.
And Peter doesn't just mean is this a guest room or an office or living room. Peter means besides the basic "label" of the room - how do you want to function in that space? Is it important for you to be able to clean it easily? Will others be using this space? Do you need to access things quickly in this space without having to move other things to get to them?
Before you start - really think about these questions and develop a clear vision of how you want to move and function in the space. Then as you start to go through the clutter, the decisions about what to keep and what to toss become much, much easier. If it doesn't support your vision - it's gone.