There's no doubt that you will be working your ass off in grad school. Between classes, research and instructing or otherwise being a grad assistant, you will be busy. Even though it's hard work, the fact that you are now pursuing your ultimate passion in life will propel you through most of the work. After you've finished working for the day, take some time to play, too. Go for a beer, watch a movie, wander around your local arboretum or enjoy some local sports with friends. Just be ready to crash in bed and do it all over again the next day :)
Now that you're out of undergrad, people are going to start expecting you to actually act your age. This means: eating plants, preparing meals and having friends over for dinner. Nearly every grad student I know is strapped for cash, so make it work by buying frozen veg (just as healthy as fresh!), avoiding pricy takeout by packing your lunch and having granola bars and dried fruit in your desk at work. Your body will thank you, and people will be impressed at your love of prunes. (maybe that's just me...)
Rule 3: Make time for exercise + sleep
Part two of taking care of yourself in grad school means taking care of your physical health. Engage in daily exercise to give your brain a break, for stress relief and to get outdoors. Join a rec sports team with your department, train for a half marathon, or sign up for a group fitness class. Pack running shoes and clothes in your backpack for quick 45 minute breaks between classes or experiments to get in a run or a weight session. And please, get some damn sleep after your day. No one likes a cranky grad student.
Between class, work, research and exercise, you're going to need a way to keep track of it all. Last year I had a paper planner and several to-do lists, but quickly realized Google Calendar is the way to go. Set up a color coded system for classes, work, experiments, bar nights and especially schedule your exercise and fit it all in. Use to-do lists and text message alerts to keep yourself on task, and check your calendar the night before to see what you need for the next day. Stay organized, stay sane.
Rule 5: Mentoring matters
Being a new grad student, you're going to have so many questions. Finding a mentor is crucial for maintaining a clear understanding of your expectations and for having someone to vent to when it all seems like it's piling up. Seek out an older grad student and take them out for a beer. Share stories of struggles and get advice on good mentors, classes to avoid and what really rocks at your university. I was lucky enough to find my awesome 5th year grad student friend Dave and he's taught me and helped me navigate the first year of grad school. You can't go it alone!
Finally, the most important part of grad school is to not lose sight of yourself. In the first weeks, you'll be meeting so many people and it's hard not to compare yourself to them. Remember that everyone got into your program because they deserved to, and for the most part, everyone is entering on the same level. Try to keep sane by avoiding the comparison trap, walking away from situations where people are being overly cocky or negative, and finding a solid group of chill people to hang out with. You were admitted to the school for a reason: the department has faith in YOU.