I remember listening to a student who was complaining about the lack of inspiration that they felt was and that this was the reason they could not complete the assignment. The instructor stood there giving his full focus to the student. Nodding while they went on at length about how they were just not feeling it. When the student had finished their diatribe the instructor took in a long breath and told them in a reverent tone "Well then, you must never leave the studio because you do not want to miss that moment when the Muse is upon you." He then turned and walked quietly away.
Occasionally the spirt does strike without warning, however for quite a few of us we invite it in. I have created several little Muse call rituals myself. I won't bore you with the details, but the biggest one is showing up. I show up in the studio and know, Muse or not I am going to be there for a minimum of one hour. So I may have to sweep, rearrange paints, clean palettes or what ever until it arrives.
There are days when it doesn't arrive. Those are spent stretching canvas. Filing paperwork. Throwing away things that are no longer of use. But more often that not, I pick up a pencil or a brush and make a mark. Then another, and then I find that I have been transported and painted for several hours, and this would have never happened it I hadn't shown up.