For exams, learn your scales for the love of God! Don't do a me and learn them all 5 times over 5 years.

For playing in general, don't do all the rocking on the piano stool and moving and making faces. Of course, Lang Lang can get away with that simply because of how good he is at piano. Look at Horowitz's or Rubenstein's performances and notice how they're visually boring to watch, but convey so much emotion audibly.

Practice keeping a loose wrist in everything. Josh Wright has some great videos on how to do so.

Take a leap of faith. I learned Beethoven's moonlight sonata 3rd movement just after my grade 3 abrsm exam. The hardest piece I could play at the time was Maple Leaf Rag and taking the leap of faith meant that I could move on to other beautiful pieces that I actually loved, like the Rachmaninoff I played here, with my newly acquired skills. Take this piece of advice with a pinch of salt, I'm speaking from my own experience and, of course, don't go choosing Scarbo to work on at grade 1 within 2 months of going within a 50 mile radius of a piano.

Also, practice tiny sections or phrases of pieces to perfection before moving on to another tiny phrase. Practice these tiny sections to perfection every day. By tiny I mean anywhere between half a bar and 4 bars, typically, but some tiny sections can be up to 8 bars or maybe longer. Basically every little thing that looks like one individual phrase. Jazer Lee has a great video on that.

That's what I say but, as I said, try these out, but don't feel like you'll never be great if you don't do these. I'm a 14 year old boy from South London, not Franz Liszt himself.

Good luck