You are what you know.
The most valuable thing in the Universe is knowledge stored in a reasoning system which applies it effectively.
For the moment, that means knowledge stored in a human brain.
Your brain.
One reason -- and one problem -- is that the write rate into human long-term memory maxes out at about two bits per second. (An observation for which I am indebted to Marvin Minsky, who relates that he got it from Allen Newell.)
Taking charge of your life and your mind depends critically on taking charge of your learning habits. The two critical parameters are quality and quantity.
In the end, only you can decide what constitutes high-quality information.
I look to peer-reviewed publications like Nature and Science as the best available.
After that, I look to people whose track record leads me to trust them -- being very careful that someone who is an authority in one field (.e.g, Penrose in twistor theory) often makes a conspicuous fool of himself in another field (artificial intelligence).
Avoid commerical TV like the plague! The content is optimized entirely to maximize advertising revenue, with no regard whatever for accuracy or utility. Learning from TV is the intellectual equivalent of drinking from a typhoid-contaminated water supply. You are what you've learned. Depend on American TV for your knowledge of the world, and you'll wind up as ignorant, bigoted and superstitious as the American public.
"I was born not knowing and have only had a little time to change that here and there."
-- Richard P. Feynman
Effectively understanding and navigating the universe depends upon possessing a wide variety of reliable knowledge.
Given the extreme slowness of human learning -- a billion times slower than computer information storage rates! -- this means that your emphasis has to be upon making the best possible use of the limited bandwidth available.
We are all born very ignorant, and to date none of us have had the time to change that more than a very little here and there before we die. One of the real habits of highly effective people is that they are always learning, always alert, always quick to take any opportunity to reduce their enormous ignorance by some useful, possibly someday critical iota.
Every hour that you fail to learn something of enduring value is an opportunity gone forever!
Make a point of never going to bed without having learned something new and worthwhile that day.
Install a mental alarm which goes off any time you find yourself twiddling your thumbs aimlessly -- killing precious time for no good reason.
Beyond this, treat your brain as what it is -- a tool which, like a car, takes time, effort and training to use effectively.
A mnemonist is merely someone who has mastered a particular skill -- one much easier than, say, learning to read. Just as anyone can learn to drive, so anyone can learn to remember.
In a sane society, competent mnemonists would be as common as competent drivers. In today's world, where knowledge is power and governments systematically minimize the power of the populations they control, mnemonists are as rare as hen's teeth -- but that doesn't mean that you should passively submit to this malign neglect.
You don't have to train for the world memory championships but do learn the basics of, for example, how to remembering numbers effortlessly. Practice it habitually on license plates and street addresses and such when not otherwise occupied.
Learn the ancient method of loci. Establish your own and practice using it regularly.
Further reading: The best book on the subject is The Art of Memory by Frances A Yates.