My #1: Get really good at user testing so that you minimize risk every step of the way. I can't say this enough as it has totally changed how I approach startups, new features, and core assumptions (and weirdly my personal life).
My #2: If you have co-founders, make weekly rituals to ensure you stay in sync over everything else. When you get out of sync, bad things happen. This can also be applied to investors, board members, spouses, etc etc etc...
My #3: If you are making a bet write it out in detail about what the bet is, what the assumptions are, and then write in detail why you are doing each piece and examine it from multiple POVs. Writing that down over a period of weeks ensures a good thought process most of the time. Write it out as if you are giving a presentation. This has helped me in so many ways.
Those are the big ones that have been huge with startup/business/life.