In Monday Morning Mentoring: Ten Lesson to Guide You Up the Ladder by David Cottrell, the book talks about three kinds of stars when managing people. There are the superstars, middle stars and falling stars. He describes how to deal with each.
It is important to remember that superstars, or the outperforming employees, usually don't get enough attention. They need an extra push with rewards and accolades or else they will be disgruntled and leave, and according to the Pareto Principle they are the ones who do 80% of the work. It is also important for them to see that not only do they matter, something is done about the falling stars, or the employees that do not perform well enough and tend to get worse.
Contributors to Wikimedia projects
Falling stars should be paid attention to, but not in the ways you might think. Good reviews do not help them do any better. It is not encouragement for them. In fact, it makes them do worse because they start believing that they can slack off more. Training is needed from the other two stars and attention from them. It is important to get the whole team working together to keep the boat afloat.
Midrange stars need to be encouraged by the superstars and given opportunities to become superstars. A lot of them do not...they generally the glue that keeps the company together, so they are the most important stars. They should be celebrated. Superstars should be focused on them to lift them up to their level. It is important to have training available from reputable sources like Pluralsight where many experts teach and teach well.
Mentorship is an amazing opportunity to bring employees to the next level. I have had the opportunity to be involved in a program like that at my job. It made all the difference.