What is the best method for painting hardwood floors?

From my experience you would indeed have to sand to break up any glossiness, clean thoroughly and perhaps prime depending on what paint you choose.  I have had the best luck with Epoxy floor paint, this is an extremely durable paint used for porch floors etc.  However, I last painted a floor about ten years ago.  The new 100% acrylic paints may be there in durability, and they are certainly easier to work with.  Check with the pros at your local paint store and see what they would recommend.