What's your favorite piece of GM advice?

Mine is: in combat encounters, you want to roleplay the enemies, too. Doesn't matter which system it is. As the GM, you want to prevent unrealistic tactics from your combat encounters. Match your players' expectations. A pack of wolves or bandits might be smart enough to employ tactics such as skirmishing in combat or moving around the game board, but a group of mindless zombies wouldn't be able to think like that. Not every creature is capable of powergaming. I see this mistake a lot from new GMs. This can and will affect the difficulty of your games, and in a system that heavily relies on tactics, might result in an unintentional TPK.