What is 2/3's Rule? Ok, seriously it's not religiously a rule, just a rule of thumb that I've always used. In my readings and experience I've found that you have to leave some room for back and mood setting material. You don't want the ENTIRE image to be of the subject, you need some filler. Your eye can't focus on the entire image anyways.
Basically means to get the subject out of the center of the picture.
People: Imagine your view finder separated into thirds, and then try to position the models eyes 2/3 over from either side, and 2/3 up from the bottom (or top) of the picture. It works exactly the same way whether you are shooting close in to the face, or full length. Make sure you focus on them first before moving away or you will get a picture of the wall. I've never done it before I promise. ;-)
Scenery: Find your object of interest and make sure you focus on it first. Very important to focus then move out from the object and get other interesting features around it. This makes for an excellent photo plus the surroundings.
I took that image and divided it, how close did I come to 2/3?
Next, I make the picture be exactly 2/3 with some cropping and editing it's a perfect photo.