Here is another exercise offered by Sri Chinmoy to help develop concentration:

If you want to develop the power of concentration, then please do this. Before you concentrate, wash your face and eyes properly with cold water. Then make a black dot on the wall at eye level. Stand facing the dot, about ten inches away, and concentrate on it. After a few minutes, try to feel that when you are breathing in, your breath is actually coming from the dot and that dot is also breathing in, getting its breath from you. Try to feel that there are two persons: you and the black dot that you have made. Your breath is coming from that dot and its breath is coming from you.

In ten minutes, if your concentration is very powerful, you will feel that your soul has left you and entered into the black dot on the wall. At this time, try to feel that you and your soul are conversing. Your soul is taking you into the soul's world for realisation and you are bringing the soul into the physical world for manifestation. In this way you can develop your power of concentration very easily. But this method has to be practised. There are many things which are very easy with practice but just because we do not practise them we do not get the result.

(Source: Meditation: Man-Perfection In God-Satisfaction — by Sri Chinmoy)

Concentration-power
Is
The strongest power
That silences
Uncomely thoughts
And
Wandering mind.
— Sri Chinmoy