Read something in a different genre than you usually do.
Go to an art museum.
Write in a different place.
Take a modern dance class.
Do something that makes you consider your world, the world, differently than you would normally. Do something that makes you ask questions. Look at art that makes you uncomfortable and then ask yourself why it makes you uncomfortable. Ask yourself why a lot.
Live in a state of constant questioning and keep blurting out answers until you find one that sticks... until you find one you can't get out of your mind, then follow it and see where it leads you.