Just like de-cluttering your house, de-cluttering your brain means that you first have to go through a process of awareness. Figure out what you're holding onto mentally that is just taking up space and not leaving room for creativity to spark.
Inventory your mental clutter
Plan the de-clutter process
Take Action on the de-cluttering
Continuously clean up your thoughts
Finish your un-finished business
Enlist new positive daily habits that help your thoughts to stay organized.