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 same home, clutter collects 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, garages, attics, and basements. You want as much open clear space as possible, so every extra little thing needs to be cleared away.
Take a step back and pretend you are a buyer. Let a friend help point out areas of clutter, as long as you can accept their views without getting defensive. Let your agent help you, too.