I was happy to have recently found two places to find Robert Schumann's Rules for Young Musicians in English online, both in pdf version and on everything2. Schumann's rules for musical development are no less apt now than when he wrote them. Here are some of my favorites:
4. Play in time! The playing of many virtuosos is like the gait of a drunkard. Make not such your models.
11. You must not only be able to play your little pieces with the fingers; you must be able to hum them over without a piano. Sharpen your imagination so that you may fix in your mind not only the Melody of a composition, but also the Harmony belonging to it.
31. If all would play first violin, we could get no orchestra together. Respect each musician, therefore, in his place.
36. For recreation from your musical studies, read the poets frequently. Walk also in the open air.
50. Do not neglect to hear good Operas.
59. Look about you well in life. as well as in the other arts and sciences.
61. By industry and perseverance you will always carry it higher.
68. There is no end of learning.