I'm a fan of the two-track approach. Spend part of your practice time perfecting the mechanical components of the piece -- notes, rhythm, articulation, fingering, and building up to tempo. Then spend the rest of the time taking what you've already learned and bringing it to life.When you first get a new piece, you'll spend most of your time on the technical side. As you master that, spend more and more on the interpretative side.If you don't have time for both, do fewer pieces, break them into smaller chunks -- or get a new teacher.