First:
Write as though everyone you know is dead.
I know I've heard this before, but hearing it again helped reassert its importance. Are you writing from yourself or writing as guided by a perceived audience? The counter-question being: is your work completely self-indulgent?
Second:
Amateur musicians practice a piece until they can play it right; professional musicians practice until they can't play it wrong.
It's tempting, when practicing a difficult passage, to move on to the next difficult passage after a few successful run-throughs. Instead, phrases must be practiced hundreds (etc.) of times alone and within the whole before they're actually successful (the counter-rule being: don't let the passage become mechanical). I had never heard the can't-play-it-wrong quote before, but it's a good consolidation of the intent.