The approach has to be deductive most of the time.
You don't need to attack a drawing at once without any working imagination of its finish stage. When you look at the blank sheet, do not draw at once. Let your mind picture it first then draw from boundaries. BIG. Layout the scene first to define proportions and scale.
Deductive.
After you made the basic layout, that's the time you do details. When you do it and you think you get tired and confused, leave the room first and drink, or divert your attention. Don't push too much when you are so frustrated to imagine. After a short break, get back... somehow idea will start coming in.
One of the secrets is to keep yourself always "artistically inspired." A trip to an art museum is usually effective.
