To learn about subject placement in an image I chose to use a canoeist on the river. Unfortunately some of the images weren't as sharp as I would like, but to learn about subject placement I was pleased with the results.
Here is the first image with the subject in the centre of the frame.
As suggested having the subject in the middle of the frame does seem to negate the background/rest of image, by cutting a hole in the middle of the image. Consequentially the rest of the image here just appears messy and may as well be cropped from the image. f/5.6, 1/800 sec, ISO-640, 190mm.
I have put the rest of my photographs in order of preference from worst to best, with my ideas about each below the image.
Having the canoeist in the top-right or top of the frame seems to do nothing for this picture, with the majority of the image just uninteresting. f/5.6, 1/800 sec, ISO-500, 190mm.
Placing the subject in the left of the image does make the river seem slightly more interesting as I can see where the canoeist has been, but the subject draws the eye to the left and most of the right of the image is lost or not noticed. f/5, 1/800 sec, ISO-800, 190mm.
Here the subject is in the bottom right which seems to make for a better image as the eye follows his course down the river and more of the overall image is taken in. f/5, 1/800 sec, ISO-400, 190mm.
The final image and my favourite. Placing the subject in the centre bottom of the frame really drew attention to the background as well as the subject, and gives an impression of a large and daunting river which the canoeist is facing. You can start to imagine how it might feel to be in the canoe. f/6.3, 1/800 sec, ISO-400, 300mm.