I'm very grateful that over the years I've learnt more and more about how to practise pieces, and to my mind there's nothing more valuable to learn for a pianist. Here are my tips:
1) Try to keep mentally and physically relaxed at all times when practising. Practising when you're wound up is actually harmful rather than beneficial, so take a break if you get frustrated and come back to it, maybe to a different piece or different part of the piece.
2) Think of different ways to practise. There are always more different ways than we initially think of, such as playing just the harmonies of a piece (this really gives you a feel for its structure), dividing up different parts differently between the hands, playing in different octaves and different keys, 'miming' one hand while playing the other at normal volume, as well as classic things like very slow practice and separate hand practice. Finding a new way to practise a piece or a difficult passage is far more worthwhile than repeating the same kind of practise on a piece over and over again!
3) Quality of practice is more important than quantity. Admittedly there is an element of repetition involved in practising, but don't just work through a routine, or play through mindlessly - always keep thinking about the music, what it means and how it should sound, and whether there are other ways you can approach the business of practising!