Focus on the task at hand: If talking with someone: ask questions; if reading a book or a report: ask yourself how you would summarize what you just read to a friend or to your boss.
In general, avoid and/or eliminate distractions. Tune out everything else. The harder the task, the more important it is to tune out distractions.
Do not try to double-task, this will increase your errors and divide your attention. Attention is limited. When you try to do several things at once, you necessarily have to divide your attention and thus concentrate less on each individual tasks.
Use meditation. Several studies have shown that meditation can be a good brain training tool that affects especially attentional / concentration skills.

Focus on the thing you are trying to concentrate, avoid getting distracted, try to be physically fit by regular exercise and balanced diet, yoga would be good to increase power for mental concentration. Avoid 'nasha' giving elements like drugs, alcohol etc.

Meditation will help to improve concentration power.

Taking interest in the subject and doing with understanding will also helps.

Will power is most important.

Draw a black circle with 1 inch diameter on a clean sheet of white paper....Fix it on the wall just on eye-level on sitting posture.....sit unmoved .... and look without twinkling of both the eyes into that circle.......... till that circle fades away and become white.....practice for some days .... U must get tremendous result......

Practice meditation and 'yoga' kind of activities...!

Meditation