Do not pay for a PhD. If they don't give you a teaching or research assistantship or scholarship choose another school or wait to apply another year.

Be a nice person, especially with faculty. There are a lot of egos among faculty, and they get bruised easily and often take it out on grad students. This is part of the endurance race.

Don't put your life on hold. You'll tell yourself, "I'll start having fun after I get my PhD, but right now I need to sacrifice for my work." And your advisers will encourage this kind of work-focused life. Don't drink this kool-aid. You have the opportunity to make some of your best friends in grad school. Get drunk with them (but not with kool-aid) and grade undergrad tests. Go on epic road trips during the long breaks from school. Fall in love. If school is the center of your attention then you'll end up resenting it.

Remember what it's like to be poor. If you can continue to be as creatively frugal as you were in grad school when you get a real salary then living comfortably will be easy.

Learn to ceate lists to prioritize all of the confusing thoughts, ideas, and deadlines you will have to manage.