Ask HN: What do you wish you knew when starting your business?

A friend of mine was contacted by a local chamber of commerce to help put on a "Entrepreneurship/Startup 101" course. Most of these, at least the ones that I've been to, only teach how to raise significant sums of money from VC's a la the 1990's dot com boom and are taught by washed up VC's themselves. In other words, most of these things are worthless.

Which is why I ask: what do you wish you knew early on that would've saved headaches? So far, our list includes: choosing cofounders, rapid prototyping, "hustling" (talking to customers immediately, and selling your product quickly to get cash in the door), and Delaware incorporation. What else?

Our target audience is hackers, so we don't plan on focusing on actual product creation extensively, other than the mindset of gauging need via talking to customers and rapidly creating a simple solution around that.