I think it is a bit of a myth that we only use a fraction of our brain. The brain uses up a lot of energy, as do some other organs such as the stomach. In the survival of the fittest, it would make sense to have an energy-hungry organ and not use it to its full potential.
That said, I am trying to get my brain to be better by doing mediation. There are quite a few scientific studies that show that is very good for you. This is the website I use (which is free for the first ten days):
As Chris says, it’s actually a myth. We use most of our whole brain, most of the time. The interesting thing about cognition is how holistic it is- that is, it uses the entire state of the brain to generate every thought. That is part of how the brain is able to be so complex, and hard to understand- unlike a computer. Brain-scanning technology is now letting us see just how active the whole brain is for even the most basic tasks.