A few aphorisms I've been working on over the years:
Most things are easier than you think they are to do: but harder than you think they are to do well.
(Really...pulling an engine in a car, programming in C++, mixing a live band...so many things seems so very complicated until you actually try it. But then, after you've done it a bit, you realize how really skilled the people are who do it for a living, and how much more you'd have to learn before you stopped being an amateur.)
You have to have a decent grounding in a subject before you can appreciate how completely ignorant you are in it.
(The more you learn, the better you understand how much more there is to learn. Only two kinds of people believe they have mastered a subject: and one kind is very, very rare. The other is, unfortunately, quite common.)
You can judge how close the scenery is to completion by how long it takes to get from one side of the stage to the other.
(As doors, masking, and prop tables get installed, you can't just stroll from one wing to the other.)
With the right mics in the right place the mix is basically done. With the wrong mics in the wrong place it is basically done for.
(If you set the mics right, turn up the faders and it will already sound great. If the mics are not set right, you can be playing with corrective EQ for days but it will never quite be as good as it should be.)
From Joe Ragey:
"Nothing is temporary in theater."
"It's going to be painted black; no-one will ever see it."
"If you did this in Chicago, do you know how much it would cost?"
Original composed music takes ten hours per finished minute. Sound effects are comparable.
(Okay, that's really more a rule of thumb!)
Borrowed from a book on aviation;
"You can't use the airspace above you, the runway behind you, or the fuel that's still back in the truck."