Be Here Now
This deceptively simple strategy is probably the most effective.

When you notice your thoughts wandering astray, say to yourself

"Be here now"

and gently bring your attention back to where you want it.

For example:
You're studying and your attention strays to all the other homework you have, to a date, to the fact that you're hungry. Say to yourself

"Be here now"

Focus back on subject with questions, summarizing, outlining,

and gently bring your attention back, and continue this practise, repeatedly. It will work!

Do not try to keep particular thoughts out of your mind. For example, as you sit there, close your eyes and think about anything you want to for the next three minutes except cookies. Try not to think about cookies...When you try not to think about something, it keeps coming back. ("I'm not going to think about cookies. I'm not going to think about cookies.")

You might do this hundreds of times a week. Gradually, you'll find that the period of time between your straying thoughts gets a little longer every few days. So be patient and keep at it. You'll see some improvement!

Do not constantly judge your progress. Take it easy on yourself. Good practice is enough to say that you did it, and that you are on the road. The mind is always different and the practice unfolds over time with many ups and downs.

Worry or Think Time
Research has proven that people who use a worry time find themselves worrying 35 percent less of the time within four weeks.

1. 
   Set aside a specific time each day to think about
   the things that keep entering your mind and interfering with your concentration.
   
2. 
   When you become aware of a distracting thought,
   remind yourself that you have a special time to think about them,
   
3. 
   Let the thought go,
   perhaps with "Be here now,"
   
4. 
   Keep your appointment
   to worry or think about those distracting issues
   

For example, set 4:30 to 5 p.m. as your worry/think time. When your mind is side-tracked into worrying during the day, remind yourself that you have a special time for worrying. Then, let the thought go for the present, and return your focus to your immediate activity.

Tallying your mental wanderings.

Have a 3 x 5 inch card handy. Draw two lines dividing the card into three sections. Label them "morning," "afternoon," and "evening."

Each time your mind wanders, make a tally in the appropriate section. Keep a card for each day. As your skills build, you'll see the number of tallies decrease
Maximize your energy level
When is your energy level at its highest? When are your low energy times?
Study your most difficult courses at your high energy times. Sharpest early in the evening? Study your most difficult course then. Later in the evening? Work on your easier courses or the ones you enjoy the most.
Most students put off the tough studies until later in the evening when they become tired, and it is more difficult to concentrate. Reverse that. Study hard subjects at peak energy times; easier ones later. This alone can help to improve your concentration
Visualize
As an exercise before you begin studying, think of those times when concentration is not a problem for you--no matter what situation. Now try to feel or image yourself in that situation. Recapture that experience immediately before your studies by placing yourself in that moment.. Repeat before each study session.
Portions adapted with permission from
Help Yourself. http://www.k-state.edu/counseling/concentr.html University Counseling Services, Kansas State University.
"Be here now" corresponds to Bhuddist insight strategy