"The great enemy of clear language is insincerity."
– George Orwell
Write as if you were talking with a friend. Be sincere, direct, straightforward, and helpful.
Treat readers with respect. Talking down to them with pretentious prose is the quickest way to lose their interest.
Sincerity encourages learning. It tells participants that you are on their side, and makes it "okay" not to know.
Avoid qualifying your statements with the words, "simply" or "easily." Being told that a lesson is simple or easy intimidates users who find it difficult. Instead of telling a user that something is easy, make the lesson easy to understand.
Humor ages quickly. Do not try to be funny or cute. You are writing for a sophisticated audience. They will refer to your work over time. Users reading a humorous item the first time may chuckle, but by the third time they will be annoyed. While there are publishers who have succeeded in marketing humorous technical books, their products are not targeted at design professionals or intended for regular use.