This is an even more common question than "how do you know when a painting is finished?" My answer, without trying to circumvent the question, begins before I ever squeeze a tube of paint on my palette. In fact, it begins with the ritual of cleaning up after yesterday's efforts. I like to start each day with a clean slate regardless of whether-or-not I was successful the previous day.
First, I throw away the old palette paper, and replace it with a clean sheet. If any usable paint remains, I transfer it to the new sheet. I put all the used tubes of paint in their proper place arranged by color. Then I replace the dirty brush water with clean water. I put away all the brushes, arranged by size. Then I grab a broom and pick-up whatever dust or dirt has accumulated on the studio floor. Finally, I get myself a cup of freshly brewed coffee.
These rituals free me from yesterday, and give me new hope for today. Organizing my work space also focuses my mind and reduces my growing anxiety. This allows me to be efficient, and reduces the frustration of not being able to find things easily. Replacing yesterday's paint tubes makes it more likely that I will use a different set of colors.
By both orienting and disorienting myself, and arming myself with a cup of coffee, I am finally ready to take on the task of working directly on the painting.