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).
To do that which we are about at any moment with full attention, thoughtfulness and commitment to do the right thing, in the right way, for the right reason is the best exercise to develop spiritual concentration. It is how we do the immediate moments that determine how we do the hours, days and years. Do the moments with full awareness of and implementation of God’s will and everything else will work out. The result is improved concentration and appropriate outcomes.
Doing now,
Z gardener
Good Morning Garden 
Love the jackknife analogy…a tool for every problem, just like God, who has answers to every problem, available grace for every situation, divine strength to meet every obstacle. If we cut down on mindless reaction time and keep focus on our “jackknife” our results will be phenomenally different: Kingdom living.
It is good to have you back in the Garden. Focus is the key word. It is what we think about, how we think and what we choose to do about those thoughts that makes all the difference.
Z