For those of us who feel that the 'rule of thirds' is cheating, this link may be of interest:
http://photoinf.com/Golden_Mean/photo-adjuster.html
The nice thing is that you can upload your masterpiece and see how it fits. You can even adjust the margins. I uploaded an IR flower shot and here's how it looks:
The shot was originally composed and shot with no particular rules in mind. The tool shows that the main in-focus flowers should be lower and slightly to the left, bringing them into the Fibonacchi curve. Or, if my intent was to show the largest in-focus flower, my composition was accidentally spot-on That is to say, the center of that flower is very close to 0.618 of the width (towards the right) and very close to 0.618 of the height (towards the top).