I remember when I was an upper class undergraduate composer who was beginning to look at graduate schools. I sent an email to University of Michigan composer William Bolcom asking if he had any advice for a young composer who was preparing for graduate studies in composition. His reply was curt: “Go study Beethoven.” Not the response I was expecting.Arnold Schoenberg, in his book Fundamentals of Music Composition spends most of the time discussing examples taken from Beethoven! Mulling over that is a nice antidote to those absurd flights of fancy such as Alex Ross' recent essay/review on Beethoven that throws up all sorts of dust, but tells us virtually nothing about why Beethoven is such a good composer.
Luke mentions some thoughts of the compose Tristan Murail as well:
Murail, too, implies that composers who are harmonically deficient or indifferent would be well served to look at music of the past, especially considering how “harmony relates to form.”The relationship between harmonic structure and phrase structure in Classical Era music is very striking and I have looked at that quite a lot, especially in my numerous posts on the Haydn symphonies.
Let's listen to some Haydn. Here is an early symphony, No. 34 in D minor, with a very long slow movement in D minor followed by three short movements in D major. It was written in 1765: