There are several techniques used by practitioners to develop concentration. The techniques are different for different purposes. For example some practice concentration to get rid of insomnia, some do it to get rid of depression, some do it to control their emotions but there are some that use it to control their subconscious.
The techniques may also differ for example some listen to classical music, some concentrate on their breath; some enchant the mantras, some focus on the ticking of the clock etc. Several concentration camps are also organized but the prime focus of most of them is to let thepractitioners attain the ultimate peace.
The power of subconscious if brought under control may do almost anything. Then it is up to the user up to what extent he uses those powers. Try to remember Uri Geller’s amazing powers. Hypnotism is also based on concentration techniques. If the purpose is to gain specific powers then the practice should be specific.
I have read somewhere that a man named Peter practiced concentration when behind thebars and then developed this to such a level that he could disappear. The police caught him but could not restrict him behind the bars. He appeared on streets sometimes and later no one knew where he disappeared to never appear again.
To practice concentration set the target first. For example if you want to move the things by your will. Start ordering the things. Remember not to restrict yourself with limitations of space, crowd or light and dark etc. Order the door to open. When ordering, do it with all the focus that you could apply upon.
At first it would not go your way as you have fed in your mind from the past 20, 30 or 50 years that the door doesn’t open that way. It would take time and patience to develop the power of mind. To break the habit of years at least give a few months for some supernatural stuff.