I’ve done this drawing ‘Hoverfly on Bindweed’ from Mike Sibley’s tutorial on negative drawing. I had read this a long time back and have used the understanding gained (separation of layers, creation of elements connecting layers, small shapes left in background to create a realistic look) in my own works. But something about this seems so magical and mysterious, I keep returning to it again and again, so finally I decided to do it myself. I really admire how he creates his realisitic works without depending too much on references. This one is composed from several different photographs. His complete tutorial along with the line drawing to trace or print is available at http://www.sibleyfineart.com/workshop_resources.htm. Its terribly useful for practice in negative drawing (having to save whites can be very frustrating for watercolor artists, so having this skill makes that less frustrating), as well as for distinguishing close tones and half tones, have a go at it yourself if you want to improve your realistic work. Its easy to see from this, why it is often said that values are what make a painting and not the colors.
Watercolor Journal 