There is another, more fundamental lesson, that I learned during my (failed) PhD - make sure that the environment suits you. By this, I mean do some research beforehand about the supervisor and the alumni. If possible talk to one of the other PhD candidates in their department and find out if you are compatible with the working environment.

This could be hard to do such early in your life, as one does not have much experience. Usually it falls in one of two categories - either you are someone that can do the work but needs support and guidance, or you prefer working on your own, in which case a more hands-off supervisor would be OK.

If you are of the former type and find yourself working for a supervisor that doesn't offer much support, it will be very hard to finish anything, and most likely you will become demotivated and drop out. Likewise, if you want to try things on your own but your supervisor wants to dictate where to go next, there will be a lot of conflicts and even the possibility that they block the thesis until it is done their way.

Having other PhD colleagues around and bouncing ideas off of them is worth its weight in gold, make sure that there is at least one that is working on something similar as you are.