Be sure science is your passion, not just a randomly-chosen degree.
As a student, think about the concept on which you want to work, not just the method. Look at your scientific training as a way to solve problems.
Be flexible and willing to take some risks. It won’t be a rewarding career if you are simply following the money.
And – something I learned from all my mentors -- don’t forget the teaching and communication missions of your work. The work I’ve done would not have happened had I not also been teaching, interacting with students, and explaining to students and colleagues what I was doing.