Put everything you don't need in to storage, a buyer is more likely to fall in love with your house if they can actually see the size of the rooms. You will sell your house more quickly and for a better price without the clutter.
Less to clean and and organize when you have a viewing, people won't feel overwhelmed by your possessions. Making a check list helps and is one of the easiest way to start. Just pick 5-10 items and make a start, you will be amazed how quickly the momentum builds and how quickly you notice the deference.