Here are some options:
1. Perform your repertory at every opportunity. Not only official performances. This means that you have to be organized. List your full repertory. Now create a number of programs with these pieces. If you have only 5 pieces in your repertory, that is it. But if you have one hundred, you may have to create 20 programs to cover them all. Now go and play for everyone: friends, family, charity, and of course official performances. Make sure you start with program no. 1 and only play it again once you played program no. 20 (assuming you have 20 programs). This will not only allow you to cover all your repertory on a regular basis, as it will give the impression that you have a huge repertory, since no one ever hear the same piece twice, unless they attended your last 20 performances. As you perform these pieces be on the lookout for the ones that may need some polishing, and polish them as needed.
2. This is a great learning strategy. It applies only to pieces you have just learned. After you learned your piece, drop it for a couple of months. If you forget it, it is even better. Now go back to it, and learn it again from scratch. Pretend it is a complete new piece. There is a huge temptation to cut corners, since you already know the piece. Don’t. Really relearn it as if it was the first time ever. The learning will be faster, but you may notice that certain passages are actually as difficult as the first time around. Concentrate on these passages. Once you have re-learnt the piece, drop it again for a couple of months. Then relearn it again. By the third or fourth time you do this, you will not be able to forget it anymore. It will be yours forever. And if you paid attention to the passages that are problematic, by the third or fourth time they will be as easy as the rest. The problem with this strategy is psychological: No one wants to do it. But it is definitely worth it.
3. Make a note of your repertory pieces so that you work through them intensively every couple of months (or weeks). Dino Lipatti used this approach. He knew exactly what he would practice or learn each month five years in advance. I personally don’t like this approach very much, because as your repertory grows, it takes more and more time you could be learning new repertory.
4. If you teach (this is my favorite) assign the pieces you want to polish to your students. This way as you teach you will be practicing (during the lessons, not for the lessons). Which is one of the reasons I never have to practice scales: I practice them by teaching them.
5. Finally, sometimes it is good to drop a piece much played for a couple of years. When you go back to it, you may see if with very different eyes. And sometimes it is just good to drop a piece (I don’t think I will ever play the Fantasy Impromptu ever again – or Fur Elise he he).
Be warned. You may not like what is written below.
After mastering a piece to your satisfaction, abandon it completely for 6 months or even one year. Your aim is to actually forget the piece.
Once the piece has been totally wrecked from sheer negligence, relearn it from scratch . And I mean from scratch. Treat it as a completely new piece. Since this is a piece you once knew, the temptation to cut corners when relearning it will be overwhelming. Resist this temptation. Go through all of the phases of the process of learning a new piece pretending you have never seen this piece before.
Even if you do that you will still learn the piece on a fraction of the time it took you to learn it the first time.
After you completely mastered it a second time, play it for a while, and then neglect it again, repeating the same process all over. You will see that the third time around you will relearn it even quicker.
Eventually by the 4th or 5th time, you will simply know the piece so well, that no amount of neglect will result in you “loosing” it ever.
Trust me, once you get to this level of mastery of a piece, piano playing becomes like riding a bicycle: you will never forget a piece.
The secret is now in planning. You must plan your long term goals (5 years) in such a way that you are always relearning some old piece from scratch.