For your wedding this weekend, try setting your autofocus points to just the central one. You can do that on the 40d by pressing the button at the top right of the back of the camera (rectangle with five dots in a cross shape icon next to it) and then using the wheel to choose the focus point you want on the top LCD screen (choose the centre one).

Video here on how to do that.

Then when you want to take a picture, line up your central focal point in the view finder with the item you want in focus (Bride or Groom's eyes), half press the shutter button, you'll get a green dot in the view finder confirming the lens has focused and then recompose the image (with the shutter held down) to the one you want to take, and press the shutter.

That way you are in control of where the camera is focusing, on the most important part of the picture.