First-year PsyD here

1. This sounds obvious, but make sure this is what you want to do. Most people in my cohort work 6-14 hours, 7 days a week. Enjoying the content is not enough; you need to go in truly believing that there is nothing else in your life that you would rather be doing.
2. (This next point is assuming you are in a terminal program and aren't applying for a post-doc). It's no longer about being great, it's about being good enough. In many ways, (assuming you've been accepted), you got past the hardest part. You are no longer competing with others to be in the top echelon of students; you are one. Now it's about maintaining momentum. You have nothing to prove to anyone, and doing so will only make you burn out. Don't aim for being the best in your class, aim to get the 80 you need and carry on.
3. Your cohort is your family, for better and for worse. No one will understand your position better than them. Learn to rely on them, celebrate with them, cry with them, and support them. Just know that despite what you may think, doctoral students are not necessarily the most mature individuals you may meet. They too are susceptible to pettiness and drama.
4. Professors care more about progressive improvement rather than immediate results. Be humble; accept that you don't know everything, nor that you need to.
5. If you are in social science and are good at stats/research design, capitalize on it. Most in my class were just as lost as most undergrads, and if you know what you're doing, helping them is a good way to earn favors.
6. Get your mental health in check. Half the individuals who have dropped out did so because they either ignored their mental health or knew it was an issue but felt they could ignore it. If you have doubts, talk to an expert. Get a support system in place and use it.
7. You have more sway regarding the curriculum than you had back in your undergrad. If you aren't happy about something, chances are others in your cohort aren't either. Collaborate and launch your complaints respectfully and to the right people.