Use your pencil instead of your tape:
Early on in my carpentry career, I mismeasured an expensive baseboard and cut it too short.
Instead of shouting, "You're fired," my boss just said, "Don't use your tape measure unless you have to."
He was right. Holding trim in place and marking it is always more accurate than measuring, often faster and it eliminates mistakes.
This is good advice for other types of carpentry work too, like siding, laying shingles and sometimes even framing.