If one of your goals is to learn to play music (or improve your playing), then this advice from musician Dick Hensold on how to practise strategically — with a limited amount of practice time — is for you. The key is what he calls "proper cultivation" of your technique.
So what is "proper cultivation?" In short: accurate, consistent repetition, while maintaining perfect technique. In long: see below!
When practicing to increase the speed of a given technique or piece of music, practice a short selection slowly, but not so slowly that it changes the entire character of the tune, simultaneously paying VERY close attention to:
1) maintaining perfect technique (defined below);
2) accuracy (defined below) ...all while striving for
3) consistency in performance (defined below).
Choosing a short selection — anything from a few notes to eight measures — will help you closely monitor your progress as you practice. By "perfect technique", Hensold means the technical way you should play the instrument (such as moving with minimal force while holding the instrument and having your body in a neutral position). "Accuracy" means playing the music exactly as you want it to be heard — especially tempo-wise, but also in terms of intonation. And "consistency" means being able to maintain all of the above while making as few mistakes as possible. Those are the three most important areas to keep in mind while you practice, Hensold says.
Don't expect miracles right away though. Your improvement will actually come during the rest periods between practising, rather than in the practice session itself:
Repeat the short selection, concentrating on all the factors above. Do not speed up, and, if necessary, use a metronome to keep your tempo down. It doesn't take enormous amounts of repetition (Perhaps 20 repetitions? Maybe two minutes of repetitions?) to cultivate the expected improvement if your concentration is good. In other words, we must be very careful to integrate all the factors above. (This is where we come back to efficiency — we are saving practice time by not spending too much time on any given problem on any given day.) Repeat every couple of days, and there should be noticeable progress in a few practice sessions. Trust that the improvement will come in the rest period between practice sessions, not on the day of the practice itself. The good news: your technique will speed up on its own, on its own time. The bad news: if you don't start early enough, you may not get your music learned in time. Thus, people are always tempted by shortcuts, which both slow the process down and lead to sloppy playing.
The rest of the article on how to play music faster is a great read, outlining the difference in practice needs between young and adult musicians, qualities you need to learn music without a teacher and more.