Ngaio Marsh on writing - I believe her words apply to musical composition as well.

“It must have a beginning, a middle, and an end. The middle must be an extension and development of the beginning, and the end must be implicit in both. The writing is as good as the author can make it: nervous, taut, balanced and economic. Descriptive passages are vivid and explicit. The author is not self-indulgent. If he commands a good style, there is every reason for maintaining it. In an age of immensely long and undisciplined novels, one can do with some shapely ones, and in the midst of much pretentious obscurity, a touch of lucidity is not unwelcome.”