Apartment Therapy, Chicago-version, offers this clever take-along floorplan. The goal is to be able to account for all the components of your room, particularly scale, arrangement in color: "You will want to include basic information like the size and layout of your rooms, color choices (fabric swatches are perfect for this!) and sizes of existing pieces of furniture. Other things that might be helpful are window heights (in case you are curtain shopping), height from the floor to the bottom of your window (in case you need something low to fit under it), and the location of your electrical outlets and floor vents (which are things we rarely think about, but end up being a pain later on)."
Brilliant, and a perfect project for a dark and stormy holiday weekend.