Here are some examples of the kinds of questions I ask myself when I get stuck on clutter:
Are you saving an item because you equate value with usefulness even though do haven't actually used it in months or years (think containers, gadgets, cleaning supplies, clothing, toiletries, decor, etc.)?
If the item were to become unusable how would your life be affected? Would you need to replace it or do you have other items that would serve the same purpose?
If it's sentimental would a photo of it spark the same memories and emotions as the item itself?
Are you saving those favorite pants for when they fit you again even though you haven't fit them for ten years and probably wouldn't fit again anyway because your body changes shape as you age?
Are you keeping it because it was a gift and you feel guilty getting rid of it?
Is it an inexpensive item that you would gladly pay the cost of replacing not to have to dig past it for the next year or two?
Have you actually worked on that project in the past six months?
Get three boxes ready, and start picking stuff up and putting them in to those 3 boxes based on:
Box 1: Definitely being thrown out
Box 2: Don't need it, and want to consider throwing it out
Box 3: Definitely keeping
Go with your first instinct about every item you pick up, and which box it belongs in. IF you give yourself too much time to think about it you will always rationalize a way to keep it.
This is a great approach.