Rest for the Weary and the Righteous

Consistent improvement comes with consistent, good rehearsal and discipline.  Memorization and mastery come by pushing the conscious to the unconscious.  Think breathing and posture.  Or, don't think.

When the vocal instrument has become so familiarized with even one piece of music, the vocalist may focus on delivering the message, trusting that the (unconscious) instrument will perform the song as rehearsed.  

The experienced vocalist will find that one well-known song adds upon another.  Soon, many aspects of the conscious will shift into the unconscious.  It's all about consistency.