The museums are full of paintings by 'great' artists that you know in your heart you could have painted better and many more that are truly inspirational. I also like to visit galleries to see what others are doing currently. I go out to lunch with friends or cook up some 'ethnic' food that I have been wanting to try. Spicy Indian food is great for getting me out of a rut. Or I teach a new group of students ways to become creative.
Nothing works all the time, everything works some of the time.
I used to view this as a waste of materials, but now I just let it happen, crumple it up and throw it away. If I really want to get something done but can't get into my art, I focus on research or the business side of art. Which would only serve to make my artist block worse. I've also found there are days when I want to make something but can't decide what to use. I set out enough supplies to make several versions.