Just like fuel your body with food, giving it the nutrition it needs to provide you with energy, you need to provide your mind with what it needs to perform at the highest level. You also need to avoid the things that are detrimental to the health of your mindset.
In no particular order, you can read, watch, or listen to Anthony Robbins, Stephen Covey, Brian Tracy, Les Brown, Jim Rohn, Wayne Dyer, and the great Zig Ziglar. You can feed your mind Napoleon Hill, James Allen, Earl Nightingale, or Elbert Hubbard. You can pick up works by Maxwell Maltz or Viktor Frankel.
You can also study the work of Martin Seligman to understand positive psychology, Shawn Achor on happiness, and Matthew Ridley on being a rational optimist, or Simon Sinek who is also optimistic and whose work is empowering.
You can also read Seth Godin, who is a force for good and always positive.
At the same time, you can decide to avoid taking in things that are harmful to your mindset, those things that speak to your fears, provide you with the perception of scarcity, cause you to question yourself and the value you create, or dredges up feelings of anger or unhappiness.
Written and edited by human brains and human hands.