I do not give exercises to develop concentration. Concentrate on what you are doing at any time, that is the best exercise I know.
I remember when I was a boy about twelve years old somebody gave me a perfect peach of a jackknife. It probably weighed a quarter of a pound and there was a little saw in it, a little screwdriver, a corkscrew, a thing for getting stones out of horses’ hoofs, and several blades. I carried it for about a year thinking how handy it would be for certain cases that never came up, but it never lost its interest. I could always find, when I was bored, new interest when I took out the jackknife. If you make your search for God your jackknife in that sense, you will get you concentration and you will get your success.
Thus will I bless thee while I live … and meditate on thee in the night watches
(Psalm 63: 4, 6).
‘Nuff said.
Peace,
Z gardener
Good Morning Garden 
I had the same jack knife! Nice post.