Here are a few of my favorite quotes of all time, from Henry David Thoreau:

“Be true to your work, your word, and your friend.”
~ Henry David Thoreau

“Do not be too moral. You may cheat yourself out of much life. Aim above morality. Be not simply good; be good for something.”
~ Henry David Thoreau

“Our life is frittered away by detail. Simplify, simplify.”
~ Henry David Thoreau

I think the last one is my favorite of all time, and absolutely applies to design of all types.

Simplify.