Is humility. I've been training for close to a decade now and I've yet to cease learning new methods, techniques, etc. For everyone, it is extremely important to remember that you can always strive to improve both your training and your attitude towards training. I've looked back at some of my old training logs from 3-4 years ago and think, "WTF was I doing!?" Then again, I'm just glad that I was receptive to trying new things and always trying to make myself a better athlete.
The field of exercise physiology and science is a very dynamic field and is changing at an unbelievable rate. Hell, things that were thought to be true during my undergrad years have since been revised during my grad years (e.g. the role of lactic acid).
So remember, what you think is the "best" way to train may not be true and there is always room to improve.