…but don’t keep your enemies closer. Screw those guys. What kind of advice is that?!
It’s so important to have an outlet which is completely non-academic during a Ph.D. Most of your waking hours will be spent doing some kind of reading, or writing, or research, or teaching, or general worrying about the process in general.
Ensure that you have a group of people who can look to for complete disconnection from the world of academia. You don’t have to meet up regularly, but even just a few hours in the company of people who can make you ‘switch off’ will make a world of difference, and remind you that there is life outside of (and beyond) the Ph.D.
(Dogs might work too instead of humans, I suppose, but they’re pretty hard to have a decent conversation with.)