Via Matt McIrvin, Total Drek’s Unhelpful Hints for Graduate Students:
(1) It is important to realize as soon as possible that you are not an undergraduate any longer. In fact, you’re not really even a student. Grad school is more like an apprenticeship program than traditional schooling. As such, you’re here to learn by doing. Stop thinking about what you do as “school” and start thinking of it as “work” and particularly as “your career.” You’re no longer practicing for your future- you’re doing it right now.
(2) Along these lines, keep in mind that nobody is impressed with your ability to get by while doing as little as possible. If you didn’t want to do the work, you shouldn’t have come to grad school. The idea here is to work hard and accomplish a lot, not just to pass classes.
Some bits of it are more applicable to social sciences and humanities than science, but for the most part, it’s excellent general advice. If you’re thinking about, or just starting grad school, read it.