Excerpt # 2 from Things You Can Do to Become a Better Player Now by Johh McCarthy
PRACTICE SMART Don’t practice what you already know, practice what you don’t know and what you need to improve on. What develping players tend to do is learn a few songs and then when they go into their practice session they work on the songs over and over again. Once you know something your muscle memory is already there. In all my years of teaching this is one of those things I tell students to look out for becuase this will slow the learning process down. When you are planning your practice sessions out think of the techniques that give you trouble or that you haven't even brought into your sessions yet or musical pieces you are not proficient at, set your goals to learn these in your practice sessions.
This is not to say that you would not on occasion go back and practice songs you know, epsecially if you're getting ready for a gig or if you're warming up. For the most part save your pieces that you play well for performing and practice your weaknesses. By making your weaknesses strengths you will become a better player!