"Be honest about what you can and can’t do."
Yep. Got burned on this once in my short time freelancing. Got cocky and thought I could pull off implementing something in a technology I've never used under a fixed contract. Live and learn.
"Question whether you’re building the right thing – within reason."
I really appreciate this point. I deal with mostly non-technical clients who sometimes don't always know what they want. Being able to get to the core of what they're after and say no to things that would make me a lot of money but be useless to them is crucial.