Since it's so critical in a startup, how do you keep yourself (and perhaps your team) from falling prey to self-deception?
Internally, I would encourage to be aware of all the weak points of any opinion you have. Do a consciouss effort to understand where your opinion is strong, proven, with good data to support and where it is susceptible to bias, demands more data, it is "just" a vision of the future. This is somehow similar to a investment. There are always risks, that shouldn't stop you from investing, but you have to understand what can possibly go wrong.
If you already know what are the weakness of your argument, than you be so offended if someone or something contradict it. You won't fall in negation, you will acknowledge that that weak point is indeed relevant. It is much easir to change your opinion when you don't think your opinion is perfect. When come the time of anargument, and you have the feeling that you "won" that argument, remember to note down the conclusion of it and reconsider all of the points raised the next day, to make sure that your interlocutor wasn't right at least in one, small, point he made.
Externally, surround yourself with diverse people, smart ones, that you trust, that have different point of views. And create a ambience where they contradict you. The more partners, mentors, investors, employees, advisors, interested friends and family you have, less likely you are to fall prey of self-deception.
I would advise you to pay attention especially to the perception of old friends, spouse and family. You may mesmerize an investor or mentor with a great rationalization you made for your vision, but your mother will always know if are into something or if you are just kidding yourself.