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Rowland,
Firstly, use the centre focus point -- focus and recompose. So long as the point of focus isn't too far from the overall composition as a whole. Example, focus on the eyes of your subject and move camera slightly down in order to frame the subject properly. You have to be very careful with focus and recomposing; you want to keep the pivot point of the camera at your nose and move the lens down in a subtle arc so as to keep within the depth of field of the plane of focus.
If you have centre point focus point expansion available in your custom settings, enable it. I have enabled it on my body and it helps the centre point get focus.
Finally, when using the 200mm lens, ensure that your shutter speed, at the very least is 1/200s, preferably 1/320s. I know with my camera, I have to use a 1/1.5*focal_length shutter speed to ensure crisp shots, handheld without image stabilisation.
It can be done with lower shutter speeds, however. Refer to attached image (slight crop) which was shot with the following details (using the very lens in discussion here, the 200mm f/2.8L II USM):
1/160s f/2.8 at 200.0mm iso500
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