Well, I think the biggest mistake is not learning the habits of saving properly early. Because saving is a habit. And then, trying to get rich quick. It's pretty easy to get well-to-do slowly. But it's not easy to get rich quick.
They say an elephant never forgets. Well, you are not an elephant. Take notes, constantly. Save interesting thoughts, quotations, films, technologies…the medium doesn't matter, so long as it inspires you. When you're stumped, go to your notes like a wizard to his spellbook. Mash those thoughts together. Extend them in every direction until they meet.
It is when our budding hopes are nipped beyond recovery by some rough wind, that we are the most disposed to picture to ourselves what flowers they might have borne, if they had flourished . . .
Our contradictions. We are in such a hurry to grow up, and then we long for our lost childhood. We make ourselves ill earning money, and then spend all our money on getting well again. We think so much about the future that we neglect the present, and thus experience neither the present nor the future. We live as if we were never going to die, and die as if we had never lived.