The most important thing in my point of view is that you should always "OWN" the project and work like as your own. This is what I learned from my career
This is important, and IMO it's the thing that most freelancers neglect. They freelance the same way they did their last W2 job - waiting for the boss to tell them what to do.
Freelancing requires a whole 'nother type of thinking and self-accountability. It's one of the most common things I end up mentoring freelancers on.