In Clutter's Last Stand, Don Aslett states, "Getting the clutter out of your life can and will rid you of more discouragement, tiredness, and boredom than anything else you can do."
Ask yourself how many things you are holding onto just in case you need them. Make a plan to go through your whole house slowly (maybe one room a week). Don't look upon the cleaning as a chore but as a chance to let "the new" into your life. Get rid of things that are no longer of use such as pens out of ink, expired medicine, outdated foods, old perfume or makeup, clothes that might fit someday.
Remember, just because you bought it once doesn't mean you have to keep it forever.
More Ways to CLEAR CLUTTER FROM Your HOME AND YOUR MIND
The Life-Changing Magic of Tidying Up: The Japanese Art of Decluttering and Organizing by Marie Kondo and Cathy Hirano
SpaceClear: Great Website for lots of resources, ideas, blogs and daily inspiration to continue to clear space.