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Lots of good suggestions above. Vball is a very challenging sport.
I rarely get below 3200 ISO to maintain a 1/500 or faster shutter. I also shoot almost exclusively vertical frames.
Center point, servo focus, wide open or maybe 1/2 stop from it (depends on your lens). For hitting/blocking shots from behind the opposite end line, I will pre-focus as others have mentioned and then use my back button to stop focus. Although my 7d will track through the net usually, as long as I don't cross the top or bottom tape with my focus point.
One of my favorite spots to sit is right by the net behind the ref, sitting on the floor. Great for back court action and setting. Like Bill said, the setter will face the same way most of the time, so look for that and adjust as needed.
I will track players, not the ball. If you can track the ball and get any decent shots consistently, you are a much better man than me! Many like to shoot continuous. I personally get better results shooting singles and anticipating the action. But then, I played a lot in my younger days, and that helps with my anticipation too.
You will miss a LOT of shots, that is the nature of volleyball. I still miss a lot, and I am getting paid decent money for doing it! Just go with it and have fun, and you will get some great shots too.