The number one thing, based on my personal experience: you have to really love what you're doing. I mean, you have to really love it. Because I don't care if you're in top physical condition, getting a full night's rest every single night, managing your time flawlessly, eating a diet that would make a nutritionist feel decadent and slovenly by comparison, and having a daily conversation with your mom/therapist/best friend/lover/Lord and Savior Jesus Christ...if you're spending 80-100 hours a week doing something you don't care about or even hate, you are probably going to be a miserable and get burned out relatively quickly.

By the same token, spending time on something you really love is going to make the little stresses, annoyances, and difficulties life inevitably brings that much easier to handle, because you won't be expending all your emotional resources just forcing yourself to get out of bed in the morning and crawl to and through another day at a job you completely despise.