Strengthening Attention with Mindfulness Training

The faculty of voluntarily bringing back a wandering attention, over and over again, is the very root of judgment, character, and will. … But it is easier to define this ideal than to give practical directions for bringing it about.
—William James

I read this quote from William James for the first time in an introductory textbook as an undergraduate student. Many questions bubbled up. What makes attention powerful enough to influence things like our judgment, character, and will? What degrades attention? How can attention be trained to improve this faculty?