I'm reading Matt Perman's book, "What's Best Next: How the Gospel Transforms the Way You Get Things Done" It is like "GTD on steroids!" :)
One point he has made several times is regarding "core values" or "core principles." He says that one way to know whether something is a core value is to ask, "Would I still hold to this even if I was punished for it?" It really makes me think, "How may of my values would I still retain if holding it would cost something or result in punishment?"
I heartily recommend this book. I will be reading it again and again I'm sure.