The (almost) forgotten master of multitasking.

Many decades before medical science was ready to acknowledge brain plasticity, a man by the name of Harry Kahne was doing something(s) extraordinary.  A piece of chalk in each hand AND each foot, some limbs writing words backwards, some forwards all the while fielding questions from the audience, Harry Kahne was truly a marvel.  I'm sure it appeared something like a magic act, but there was no trick.  He claimed anyone could do it, even writing out guidelines for just how to learn.  Here it is, completely free:

Harry Kahne: Multiple Mentality Course

Why his methods are not picked up by researchers and used to help people recover brain function I will never know. 

Harry Kahne writing with both hands, both feet and mouth.

Looking through these exercises,  you start to see a trend.  Take something you know, such as the alphabet, and learn it backwards, forwards, mixed up; every way imaginable.  The brain improves and makes new connections the fastest when it is challenged with new activities.  Knowing only this basic premise, you can create your own mental challenges to sharpen your brain.  But start with Harry Kahne's guide.  There's really nothing quite like it out there even today.  He was way ahead of his time.