Start in greyscale, move on to colour when the composition of the image is completed.
Start by drawing some large shapes and find a eye-pleasing composition.
Mask image into layers.
Use broad strokes, don't add more detail to any part of the picture before it is required. If the image is recomposed, this also avoids details getting lost prematurely.
Lighting
Start painting occlusion, and add directional lighting later, when the light sources are determined.