Think of concentration improvement as a three-step process:

1) Learn the causes of poor concentration and decide which apply to you;

2) Understand what you can do to control these factors; and

3) Make your control habitual.

Learn the Causes/Control the Causes

External Cause: Environmental distractions: TV, chairs that are too comfortable, snacks, other people, etc.
Control the Cause: Leave or re-arrange a distracting environment. Go to a library or a classroom when you seriously intend to study.

External Cause: Noise: Music with words, conversations
Control the Cause: Train yourself to study away from others and in silence.

Internal Cause: Physical distractions: hunger, drowsiness.
Control the Cause: Plan to study when you are most alert. Eat a high-protein snack. Do five minutes of light exercise to wake up.

Internal Cause: Boredom, dislike, disinterest
Control the Cause: Find a reason for taking the class; talk with other students and the professor.

Internal Cause: Anxiety about studies
Control the Cause: Make sure you know how to study effectively. Put the course in perspective.

Internal Cause: Intimidating study tasks
Control the Cause: Break up large tasks into achievable subtasks. Do the most intimidating task first. Give yourself rewards for progress and punishments for avoidance.

Internal Cause: Daydreaming
Control the Cause: Separate daydreams from studying. When your mind starts to wander, write down the interrupting thought and continue studying. Or recall important points and then turn away from your book and continue to daydream. When you're ready to read again, do so. The trick is not to daydream and read at the same time.

External Cause: Personal worries
Control the Cause: Identify and define the problem and develop a concrete, specific plan to resolve personal worries. Talk with someone who can help: a friend, a counselor, a specialist.

Make Control Habitual

Even if you lapse into old habits of distraction and daydreaming, keep insisting that you concentrate using these controls until you can routinely concentrate well on your studies for fifty minutes of every hour.

Source:
   University Of Texas At Austin