I had this amazing boss. He had 25 years of experience in the sector covered by our software, an ERP. He knew how to be a programmer, a boss, a sales manager, a support person.
I learned most of the best practices from him: do not shout in the office, it makes impossible to work. Don't hide something to your coworkers, nobody was trusting him. Be clear with your clients, his subtle mind tricks pissed off a lot of clients. Your client needs to see an economic advantage in your offers, trying to sell gold priced shit is not a good way to stay in the market. The list could go on and on and on.

I learned what happens when you do everything in the wrong way, and I will never forget.