The Rule of Thirds is a huge compositional tool and can be applied to just about every photo. It is the fastest way to make a photo go from dull to interesting. It is also very easy to do.
If you visulize your subject in your photo you see them more likely then not centered smack dab in the middle of your photograph. This is most common the case and unfortunately most commonly the biggest mistake made when a photo is taken. The rule of the thirds sounds confusing but is way more basic than you would think. This rule can be applied either straight in your camera while you are taking your photo, or in your post processing. While I do try my best to get it right SOOC (straight out of the camera) I find it is much easier to go back in Photoshop and do it that way. I will also provide a quick tutorial on how best to do this. I work with Photoshop CS3 so I apologize in advance for those not using Photoshop. I am sure however there are ways in any program to make this work for you.
For starters I am going to show you of what a centered run of the mill image looks like so you have a good visual. It will be easier to understand if you see first hand where I am jumping off from.
BEFORE:
Now that you have seen the centered photo here is the same exact photo with the rule of the thirds applied to it.
AFTER:
See how your eye is drawn right in? When cropped to the rule of the thirds the photo is not only more aesthetically pleasing but much easier on the eye because it gives the eye a good resting point.
I am now going to show you how to acheive this in your post processing. I always do it first thing before I do anymore general editing to my photo.
First with your photo opened you want to go to your shapes tool and click on the custom shapes option. 
From there you want to go down to the shape that looks like a grid. It is 2 lines diagnol and 2 lines horizontal to make what looks like a tic tac toe board.
With that shape selected you want to drag it over your photo until it covers the entire thing. From there making sure that you constrain the proportions on your photo, meaning don't squish it, drag the corners of the custom shape layer until it rests in the top third of your photo. You want the corner point to meet on the focal point of your subject.
For a quick tip to constrain proportions easily you can hold down the shift key and click and drag at the same time and it will keep your photo from being distorted.
Now that you have the points matched up correctly choose your crop tool and crop over the border of the custom shape tool. Now delete the shape layer and you should have your newly cropped photo to the rule of the thirds.

Hope you enjoyed this tutorial!
Sarah C.