Have you heard of the Golden Mean before?
This is an aesthetically pleasing unit of division that artists and architects have used time and again. It is found in nature, and it seems to give a certain amount of comfort to those who gaze upon it. An artist' canvas that contains things grouped and divided per the Golden Mean tend to "look" better.
How can you use the Golden Mean to help you divvy up your canvas, you ask?
Your best method is to use Photoshop or Illustrator, as you can create a "Golden Mean" ruler of sorts that you can interact with while measuring against your canvas. Create a canvas that is 13in tall (Don't worry about the width for now). Then find the 8in mark and use a Guide to mark it. You canvas is now separated into a larger portion, and a small portion. Fill these two portions in with two distinct colors. I use bright magenta and a pale lime green on mine because these colors are seldom seen in painting and they are very easy to spot. Then use the crop tool to adjust the width of the canvas to your liking. You can then use this ruler to help you mark up where to place things on your canvas, etc.
If you want something more traditional, you have two options. You can create the above implement by using a traditional ruler, pencil, and folding a paper, OR you can just go here [link] and this handy website will tell you where your Golden Divisions lay.
FYI: I picked this handy tip up from a Jim Krause book titled "Design Essentials Index" from How Design publishing. The book is for graphic design artists, but I highly recommended as any artist can benefit from its concepts.