GMs, what advice do you wish you had received when you started?

For those of you who have been GMing for a while, what are some things you wish someone had told you when you were just starting out?

It can be about session prep, improvisation, dealing with players, managing expectations, building encounters, handling mistakes, or just learning not to overprepare everything.

What advice would you give to a new GM today, especially something you had to learn the hard way?