Exercise: The inner flame
Before you meditate, try to imagine a flame inside your heart. Right now the flame may be tiny and flickering; it may not be a powerful flame. But one day it will definitely become most powerful and most illumining. Try to imagine that this flame is illumining your mind.
In the beginning you may not be able to concentrate according to your satisfaction because the mind is not focused. The mind is constantly thinking of many things. It has become a victim of many uncomely thoughts. The mind does not have proper illumination, so imagine a beautiful flame inside your heart, illumining you. Bring that illumining flame inside your mind. Then you will gradually see a streak of light inside your mind.
When your mind starts getting illumined, it will be very, very easy to concentrate for a long time, and also to concentrate more deeply.