Here are some other resources about that "Rule Of Thirds" idea for composition....
This video talks about using it for photography, but it works for drawing too-
See the video here @
http://vimeo.com/14315821
AAAND
Set up the built in grid in Photoshop to make a "rule of thirds" grid for you!!! Follow these steps:
Open Photoshop's "preferences" controls... and choose "grid".
Set up the spacing of the grid lines and the division of the grid as shown here....
Lastly, to turn on and off the grid, hold the "command key" and press the ":" key...
done!!!