The difference between a "good painting day" and a "bad painting day" often comes down to mindset. This video shows a simple but effective warm up exercise to start painting with the right mindset. If athletes and musicians begin each session with stretching and warm up exercises, why aren't you? Painting is a physical and mental exercise, and you benefit from a bit of limbering up before you work.
In school you've probably grown to loathe the term 'study'. When it comes to math and history, this term is synonymous with painstaking memorization. In the case of art this term can take on a slightly different meaning. On ctrl+Paint I often refer to 'studies', and this video helps to explain my meaning. Though your version might take on a different form than mine, 'studies' are the way you improve at art.