Anyone Have Any Concentration Exercises

             I have blundered away countless games since my return to chess this year, and my concentration is weak.
             I also tend to play a lot of long games, over 50 and even 75 moves.  It
only takes 1-3 mistakes to lose and I can't seem to stop doing them.  The second
I make the move in live chess I know it is wrong but it is too late. In online
chess I get tired and don't double-check properly.
             Does anyone have any good tips or tricks or exercises too improve
concentration?  I am 46 now, and although I know I shouldn't use it as an excuse
it is harder now.  I know Tal won the world blitz at 51 but that is so unusual.
             Any ideas would be greatly appreciated.

Arent those exercisese from that book rapid chess improvment?

Also do these exercises really improve your calculation abilities?

Physical excercise can help you keep your concentration longer aswell.

I practiced and drilled double checking using games against a computer. I created a list of thematic mistakes I made in my games and from that made a checklist to check every move.  Things like...do I have any checks/threats/captures?  Does my opponent? Are there any pins on the board?  If there is a pin, does the pinned piece guard anything important? Is this move hanging a piece? etc.  Then I played against the computer, going through the checklist after every move out of the opening.  It can be somewhat exhausting to do this exercise, but I believe you will see results.