The Rule of Thirds is one of the most well known principles of composition and one of the easiest ways to improve your photo's. All you need to do is before you take the shot think about the rule of thirds :)
The above picture shows how the rule of thirds divides the frame into thirds (both horizontally and vertically) so that you have 9 parts. It identifies four important parts of the image that you should consider placing points of interest in, but it also gives you four ‘lines’ that are also useful positions for elements in your photo.
The theory is that if you place elements of your photo or even your entire subject along these lines you will achieve a more balanced and aestically pleasing photo.
In a similar way a good technique for landscape shots is to position the horizon along one of the horizontal lines.
You should make sure that when you are getting ready to take a photo you ask yourself what are the points of interest in this picture and where am I placing them?
And thats it! It's pretty easy right?
Well the last thing that I would like to add is that as with most rules they can be broken as the rule of thirds is not a MUST do but rather a CAN do.