To build a strong will power is one of the important means to improve concentration power. Always be positive and keep repeating to yourself that you can achieve anything in the world. Mind you, it is very important to feel good about oneself.
Develop the habit of speaking less and listening more. This way you will be able to learn and observe much more. Listening and watching life around you is a great way to not only learn but also enhance concentration power.
When you have a work at hand concentrate only on that. Strive hard to stay focused should be your mantra. Focus on one task at a time.
Just like a muscle requires regular exercises and work outs to strengthen it; your mind too requires mental exercises that will slowly but steadily improve your concentration. If you are doing a task and feel like taking a break in between then wait. Sit and try concentrating on the work for another half an hour. It is definitely difficult on the onset but eventually you will be able to see how you are capable of sitting for hours together with all your concentration.
Meditation is an excellent way to strengthen concentration power. Everyday take some time off your schedule and meditate on various activities around you. Like concentrating on the second hand of a clock, or focusing on the candle flame or on your breathing. All these techniques are simple enough but might require some time to master and not to forget they are very effective.
It is also important that you should know how to manage your time. Every morning make a mental evaluation of what you will be doing throughout the day and then set your priorities. And accordingly go about achieving them.
If hours of concentration is making your mind scatter from the task then you change the task and concentrate on the new task. This will give you a change and hence also make you feel refreshed.