I did a speed reading course when I started uni and yes it does work. A lot of stuff is bad habits - some others have mentioned, sub vocalisation, keeping your eye in the middle of the page and read with your peripheral vision, obvious stuff - don't slide your finger along the page, don't use a ruler (some do) and so on. I improved my reading and comprehension quite a lot, but find I turn it off when reading for pleasure versus reading to extract facts. It's well worth doing - even if it only increases your reading speed 10 or 20%.
[edit: How to skim read was another useful tool]