Remove dark curtains or shades from all windows in the studio.
Install shutters in the windows that can be opened throughout the workday and closed at night.
Place sheer curtains across the window. This will diffuse the harsh natural lights that can make working in the space difficult.
Position any desks, easels and work areas directly in front of or underneath a window.
Install a north-facing skylight in the ceiling over top of the work space. North light is more controlled than direct sunlight. The light values shift gradually throughout the day, allowing you to view work in the same manner, and not have it appear drastically different as the sun comes and goes. This is a more costly solution that requires building knowledge, but it creates a better working space in the long run.
Place at least one mirror on every wall. This will reflect all incoming light inward at all times.