My violist friend Joy Pile gave me this quote:
Heifetz said “if I don’t practice after one day I will notice, after two days the critics will notice, and after three days all the audience will notice.”
Here’s one another from the Jascha Heifetz official web site:
Practice like it means everything in the world to you. Perform like you don’t give a damn.” (http://www.jaschaheifetz.com/about/jascha-heifetz-violinist-nonpareil/)