Take care of yourself. Insufficient sleep, lack of exercise and poor eating habits interfere with concentration.
Your study area should be free from distractions. If you do become distracted, take time to analyze what is distracting you.
Make sure your study area is comfortable, but not so comfortable that you doze off. Beds and easy chairs are not recommended for study.
Time how long you can concentrate and schedule your study sessions for that length of time. Usually study sessions should last no longer than 50 minutes.
If your concentration time is very short, set a timer for 1 or 2 minutes longer than you usually concentrate and gradually increase the time to at least 20 minutes. Don't try to increase too quickly or you will defeat the purpose.
Take a 5 or 10 minute break after every study session and then study a different, non-similar subject.
Learn actively. It is easier to focus when you highlight, draw diagrams, read out loud, make summary notes, turn headings into questions and try to answer them or use recitation after reading a section.
Break homework tasks down into small, manageable chunks and work on one chunk at a time.