Which one thing you do before/after every job change that helped you in future?

Leaving on good terms and maintaining good relationships with former colleagues

(And taking good templates, etc)

Taking a break for a few months between jobs, and using that time to revise all my Computer Science basic subjects from college, revising all the technologies I've ever worked on in the past, and learning & getting up to date on some latest new technologies.

Doing that repeatedly has tremendously helped my technical skills and career progression.

For me its been getting LinkedIn recommendations from my colleagues and customers. Over a period of time it is a good read (nostalgic) and nice history for future employers to go through.

Make a note of what I don’t want to be as a person if I ever get to manage people

Noting down all the work you have done in a company before leaving. It’s helpful to look back at what you learned, where you contributed and what you need to learn more to keep growing.