Doing a preliminary sketch is one of the best ways to insure the success of a painting. In the video below I focus on keeping the design of the sketch strong by using three value zones. In this case I kept the sky and water as the lightest zone, the ground plane as the middle value zone, and the trees, shadows on the embankment (which read as upright planes) and the reflection of the trees as the darkest zone. Try it and see if it works for you.
Brad Teare April 2013