Subvocalization, or "reading in your head" is the process of pronouncing words as you read them. This artificially limits your reading speed to the speed at which you can speak, which is much lower than you're capable of comprehending. Some people even go as far as mouthing words as they read. Stopping this habit will result in a substantial increase in reading speed.
Using a pacer to keep the eyes scanning across the page at a uniform speed will also give a speed boost to most readers. Most readers use their finger, although a pen or pencil serves the same purpose. If you're reading on a computer the mouse cursor can do the job.
Most adults don't have to read every single letter in a word to read the word. Likewise, very fast readers typically don't read every word in a phrase. They're capable of reading phrases at a time.
An advanced tip to improve your reading speed is to learn how to read a line backwards. Once you can do this, you can read a line by reading one direction on the page and then read the following line as your eyes track backwards across the page. This pattern of reading will result in your reading every other line backwards, thus in a "Z."
Practicing the above techniques will cement a solid reading speed boost for most readers. Use a timer and the same reading material and see how quickly you can get through it on a daily basis. Try using different kinds of material after you're comfortable with the techniques.