This is the hardest thing for most people to do because they are emotionally attached to everything in the house. After years of living in the homes clutter builds up in such a way that may not be evident to the homeowner. However, it does affect the way buyers see the home even if you do not realize it. Clutter collects on shelves, counter tops, drawers, closets and garages.
Take a step back and pretend you are a buyer. Let a friend point out areas of clutter, as long as you can accept their views without getting defensive. Let your agent help you too.