It is better to remember the obvious than to grasp the esoteric. - Charlie Munger

Look first for someone both smarter and wiser than you are. After locating him (or her), ask him not to flaunt his superiority so that you may enjoy acclaim for the many accomplishments that sprang from his thoughts and advice. Seek a partner who will never second-guess you nor sulk when you make expensive mistakes. Look also for a generous soul who will put up his own money and work for peanuts. Finally, join with someone who will constantly add to the fun as you travel a long road together.

Richard Feynman as follows: “The first principle is that you must not fool yourself, and you’re the easiest person to fool.

What I would say is the single most important thing, if you want to avoid all the stupid errors, is knowing where you're competent and where you aren't. And that's very hard to do because the human mind naturally tries to make you think you're way smarter than you are.