What I do is pretend every time you sit down and practice is like you're practicing the day of your lesson. That makes me concentrate and practice everything very near perfectly, as I would play it for a lesson.
That's a good idea.When you learn, don't sight-read through the entire piece over and over - bite off small bits and memorize them perfectly before going on - sometimes just a line or two. It takes discipline to do that - you must resist the temptation to go on and play the rest.