The brain is often compared to a muscle, the more you use it, the stronger it gets. Ditto the brain.
However, there are a nunmber of good reads out there from the likes of Edward De Bono (I particularly like "Latral Thinking") and Tony Buzzan who writes about brain power and memory.
His latest, which I am now reading, is "The Power of Verbal Inteligence".
In many ways the way we use our brains has changed dramatically both over the years and with our individual lives.
As children we are, for example, pre-programmed to learn and have natural language abilities, with a inbuilt grammer, apparently, but equally apparently no inate ability to spell and that may be because written language and mature language is an artifical tool.
Historically, when only the elite could read and write, "ordinary" people had impressive memories and perhaps more dexterity with mental excercises.
The ability to read and write took that need away and people progressive lost the impetus to read and memorise.
In school, about the only things you needed to memorise were multiplication tables and poems (e.g. the Rhyme of the Ancinet Mariner of exceptional length). Today you are told you don't need to memorise things, you just look them up.
However, the inate abilities are all still there, just not used and hence the success of writers such as these in tapping into these abilities and showing us how we can indeed improve ourselves.
So my suggestion is to excercise the mind as others have suggested, but like everything esle, there is always an "expert" who can help us do so more effectively.
JMW