Two old saws I learned when I started working on a construction site were:
Measure twice - cut once.
All you need to know to be a plumber is that s#!* flows downhill, and Friday is payday.

When I began working on cars, one maxim I recall being drummed into my head was:
You don't change the cam without changing the lifters. And vice versa.
If you're tearing down a motor and you think you might be using the same cam and lifters again, you must keep them in their same old relationship upon reassembly.
Or else...

The one I came up with after borrowing a LARGER breaker bar when attempting to loosen some large, frozen nut was: Torque is cheap.