I was reading a book written by one Josef Hoffmann. In it, he touchs on some theories on practising, which includes,
practising in the morning, just after you wake up, citing mental freshness.
I go so far as to say play away for an hour, or a half hour even, before breakfast. never practise more then an hour, at most two. Then go do some stuff which is totally unrelated to music.
making short pauses every half an hour.
Follow the example of the painter, who closes his eyes for a few moments in order to obtain upon reopening them a fresh colour impression.don't practice systematically, eg. everyday at the same time, play the same sequences of the same studies/pieces.
The second tip helped me quite alot. Once I was struggling with a piece for like 2 hours with little success. I gave up and went to play video games for an hour. When I resume my study, it all came to me naturally.
