When a boxer wins a fight they immediately get back to the gym and keep practicing for the next one. If they didn’t they would most likely loose their next fight. The more and harder they practice the more fights they’ll win. Art is the same thing. When an artist sells a painting they get to the studio and keep making more. The more paintings they make the more sales they’ll make. Just because you have sold one painting doesn’t mean you can relax and expect your other paintings to sell too, which is why you need to keep making new ones all the time. You also need to be making new piece between each show you have which will keep your audience coming back and even grow.
As a boxer, the more you practice the more you’ll win. As an artist the more you practice the more work you’ll sell.