For most of us, waning concentration at any particular time relates to two threats to our focus on the task at hand. First, there are probably things we would rather be doing and second, the distractions at hand. A noted management authority used to refer to these distractions as "the tyranny of the urgent". These would be things like our cell phone ringing, remembering that your car has a flat tire and you are going to have to put the spare on before going anywhere, the need to stop studying to put the clothes from the washer into the dryer so you have clean underwear tomorrow. Maybe your roommate needs to borrow ten bucks. Maybe the mail has just arrived and you are expecting a check from home. Maybe you will have a deadline soon, and your essay is not written yet and only writers with this contact form can help you. Maybe you were supposed to have paid for your tickets to the spring dance by today and you have got to find time to get it done. All of these things are distractions that require prompt attention, and particularly if you are in your room trying to study, are available. The temptation is to drop everything and address these things of lesser import. Virtually all of them could wait an hour or two to address until you finished your study period.
If you are having trouble concentrating or focusing during your study periods, go back and check the parameters that were presented earlier dealing with distractions and study environments and make sure you have addressed these areas effectively. If not, take immediate steps to do so.
Reading Comprehension
Always assume that you are learning by adding new concepts onto concepts you have recently or not so recently discovered. Think about building a sky scraper and how different terms, concepts and ideas fit together to give rise to your structure and understanding of the bigger picture. Review and testing yourself on the material after reading it are keys to proving to yourself that you have a grasp of the material just covered. Summarize what you have read in your notes in your own words and review these notes regularly.
It is important to understand that you need to vary your style and speed of reading with the material at hand. Most people will "read" a newspaper by scanning a page until they find something that sparks their interest and then they will start to skim that article, not worrying about picking up the minor details unless it is something very important. At that point their reading will slow down dramatically until they understand what is being reported or opined. Likewise, technical material and important complex concepts should send up a yellow flag as you speed along and cause you to slow down to make sure you pick this part up.