Over the past 20+ years in the industry I have found that yes, saying "no" is the most important component. If you let other people to walk over you eventually

Over the past 20+ years in the industry I have found that yes, saying "no" is the most important component. If you let other people to walk over you eventually they'll complain you're not flat enough. This includes not working on job-realated stuff on your own time (which is something the employer will almost always try to cajole you into doing), and not checking work emails after work. The other two major components are sleep and physical activity. Just get that 30 minutes of HIIT cardio done, every day like clockwork, and sleep _at least 8 hours_ a night, every night.