For me, it's probably something my dad used to say in reference to drugs. But honestly, I have been able to apply it to almost every aspect of my life.
"When something begins to define you, it's probably not a good thing."
Basically, he was telling me - you can do drugs, but you've got to do them in moderation and don't let it become the defining factor of your life. I've used that for everything - drugs, fashions, thought processes/beliefs, etc.