Try keeping these seven qualities in mind as you practice. I borrowed these from Gerald Klickstein's 'Musician's Way"
He has a short article about them at: http://musiciansway.com/blog/?p=1137
4) Rhythmic vitality
5) Beautiful tone
6) Focused attention
7) Positive attitude
I like thinking of these as "improvable" rather than having a pass/fail attitude. This means a practice session can be fulfilling and successful if you end up playing more accurately than before - even if it's not perfect. You can make your tone more beautiful than before, even if it's not yet the best tone you'll ever have, etc. Excellence, in this sense, is to continually, and happily, improve from where we are. This does not mean settling for a lesser result, but rather keeping in mind that your practice is an ongoing, lifelong process.
How do we invite each one of these qualities into each practice session?