If you could dispense with a single point of advice/wisdom to a new but promising writer, what would it be? And why? 
Your best friend is in town and you haven't seen him or her in years. You have something very profound that has happened to you that your friend does not know about yet. You go to a bar and after two margaritas you begin to tell her/him this very important thing that has happened to you. Remember, you are intimate with this person. You don't have to keep up any appearances. Because you trust your friend, you can let it rip. Nothing is taboo. This profound thing is something that you must share. In fact, the actual sharing of it with your friend is a huge part of its importance.
You're going to treat this experience with your friend as if you were talking to your friend but actually you are writing. The trick is to write like you talk. Often the trouble with writing is that it sounds written, not spoken. So many of us have years and years of creative writing jammed into our heads, and as soon as we sit down to write, the sound that is creative writing starts coming out. It is an insidious sound. My teacher called it writing to the page. And you can hear it immediately. For example, any movie that has a scene where a character reads a piece of writing, the way that writing almost always sounds is how we think writing should sound, and that's what I mean when I say it sounds written.
Oftentimes, just reading your writing aloud can help you hear if it sounds like you, or if it sounds like you writing. Don't be afraid to break syntax rules. Use run-on sentences and sentence fragments. Really try anything to make it sound fresh and alive. Repetition of words. Choruses. Bad grammar. Cursing is allowed.
One of the sure signs that you have succeeded in not sounding like writing is if you, by surprise, make yourself cry.