Piano Playing Principles

You should always play from memory. Playing from sheet music is as if you proposed to someone and read it from a piece of paper.

Don't work your way through a piece of music bar by bar, as if you were in a gym. Rather walk through it like through a landscape where you know every tree and every brook and every flower.

Your fingers can be faster than your head, thanks to their reflexes, but the reflexes may deteriorate. Is it you that's playing the piano, or are your reflexes playing it?
Practising in your head is a good way to practise your reflexes. If you can't play something in your head, then you probably won't be able to play it really well with your fingers, either.
Practising in your head is also a good way to practise the music because then it's you that's playing, not your reflexes.
Don't play as fast as possible. Rather play a bit slower but with as much control and awareness as possible.
Piano playing is a hare and hedgehog game: Your fingers/your reflexes are the hare, your head/your awareness are the hedgehog. Your head/your awareness/the hedgehog should be there first at every note.

Cramming vocabulary may not be very effective, but it doesn't harm the vocabulary, either. Cramming piano pieces spoils the pieces.
A hundred repetitions in 10 days are more effective than a hundred repetitions in one day.
One repetition every hour in your head is more effective than 10 repetitions in a row on the piano.
Your memory has its own rhythm. Give your memory repetitions when it needs them.

Pieces must be practised, and they need time to develop. There is a time to work on a piece, and there is a time to play it.

The next piano lesson is no reason to practise only for it all week.

Weeks or months without learning anything new are lost time that you can't get back.

Forgetting something because you didn't repeat it in time, is lost time that you can't get back.

But there may be pieces that you shouldn't have learned in the first place, and there may be pieces that you don't want to play anymore eventually. That's ok.

Piano playing should of course be brilliant and expressive, but it should also be simple, true and natural, with as little ego as possible.

Expression is not something that you push into the music, but every piece of music has its own expression, and you have to find it and to recreate it.

Thinking has to do with awareness and music has to do with awareness, but music has little to do with thinking. You can't explain music, but you can become more and more aware of what's happening in the music and how everything fits together in the best possible way.

You don't play the piano to get points from your teacher or from the composer. Your mistakes are your problem, and you should play as it seems right to you. Not because you know better but because your awareness is the only one you have.

Music doesn't just start with a cut and eventually end with a cut, but every piece of music comes from silence and returns to silence when it's time, when you have said what had to be said.

When I was young, I had some success, and my success made me happy, and I didn't even realize that my piano playing didn't make me happy.
Piano playing should make you happy. Happiness means that you like what you do, with or without success. 🙂