I usually work from an established drawing before I lay down the under-painting in most of my work. In these two watercolors I did a dry on wet under-painting and then let my intuitive side see a scene and create it. The under-painting will lead you in a particular direction. It looks easy but it is very hard to get the perfect first layer. It helps to use paint right out of the tube and make sure both sides of the paper are thoroughly wet. This is really the most sophisticated level of negative painting and if you want to give the right brain a work-out try it!
I have two workshops coming up all about negative painting. Go to my web-site at www.paintpencil.com and look under classes.