When I am drawing something, like a detailed illustration, for someone else I will scan it
several times throughout the process to make the editing-to-come easier.
Here's an example:
step 1: apple tree
step 2: stonewall and foreground
step 3: girl on wall, separate drawing
step 4: finished wall
step 5: background
step 6: photoshopped the girl onto the wall and a butterfly overhead.