Three most important psychological principles in the videos that can be applied to business writing.

Treat every text based communication, as you would a face to face conversation. If you would not say it to their face than it should not be said through a text based message.
When communicating online, you are unable to see the nonverbal actions the other person is doing so make sure the words that are being used have a clear and definite meaning which can not be interpreted any other way than the way you wanted it to.
Give truthful compliments, humans naturally feel better when receiving compliments.