Stay capable of being amazed; certainty is a poor substitute for wonder.

Certainty is comfortable and closes the room. Once you are sure, there is nothing further to look at, and the world becomes a set of confirmed items rather than an open field. The capacity to be amazed is what keeps the field open, and it is lost gradually, through a thousand small moments of deciding you already know.

Protect it deliberately. Read something outside your competence, ask a question you would be embarrassed to ask, go somewhere that does not fit your categories. The goal is not to be less certain about everything but to keep the door from closing entirely.