One piece of advice is really hard to pinpoint. Following is what I would suggest.
I would really emphasize the importance of learning and understanding the science of exercise. Specifically, how a healthy body responds to exercise.
I would emphasize the importance of understanding human movement (origins, insertions, etc.).
I would also emphasize the importance of really learning and understanding what is an exercise prescription.
Lastly, I would encourage the new fitness professional to become proficient at exercise program design.
Really great question.
There is one thing I would add to Joanne’s list:
Realize that you are running a business and make yourself knowledgeable in this area. I have seen too many trainers who wanted to follow their heart but ended up in a financial quagmire because they underestimated the importance of being a business person.
Understand that it’s impossible to know everything and so be kind to yourself. Allow yourself the “luxury” of admitting to yourself and a client that you don’t know something, but be ready and willing to find out. No one is an expert on EVERYTHING!