Keep a journal, whether it is a notebook or a file on a computer.
Every time you see an image, a texte or an object that seems interesting, you need to register it, either by taking a picture, by saving a screen shot, or by writing a description.
The purpose is not to copy something already existing, but to make a collage (in your mind or on paper) of all this documentation. Some might call it a mood board others an inspirational board, but what ever way you want to make it, you need to start that way.
During my cross stitch designer years, I would spend hours in a store called Maisons du Monde (Homes of the World), it's a store to decorate your home. I would just look at the frames, the boxes, the furniture and imagine what cross stitch I could create on them. I bought a gardening jug and cross stitch a jug design and glued it on the jug. I bought the mechanism to make a clock and made the four seasons around it. I bought some glass soda bottles at the supermarket, loved...