For the last few years I have been exploring how to write truly harmonic music again. I think the first thing composers should shed is their irrational fear of harmony. Don't worry, even if you write functional harmony it isn't going to sound like Beethoven or Brahms anyway--unless you try to parody them. What you need to do is develop a sensitivity to harmonic function and let your instincts go to work. There are two models that I think can be useful. One is Debussy as I talk about here. One thing I am surprised by is that while, on the one hand, everyone talks about the huge influence Debussy had on 20th century music, but on the other hand, everyone avoids being influenced by his music. Dig into it and you will see that underneath the beautiful 'impressionistic' surface, he is using a lot of harmonic function to structure the music. The other model is Shostakovich, anathema though he may be to composers. He is certainly appreciated by audiences and performers. And yes, he very much uses functional harmony, though of a kind that has scarcely been examined by theorists. Another composer who continued to write functional harmony was Benjamin Britten.
The lesson that most composers seem to have gathered from the paroxysm in music that happened in the 20th century is that functional harmony is no longer a possible option. But I think that music with no functional harmony is crippled both in terms of how you can structure it and in terms of the expressive content. So let's rediscover functional harmony. We don't have to follow the old rules, we can make up new ones, or just rely on instinct. Here is another piece by Britten: