I didn't get any writing questions submitted for the last writing post of the year, so I'll do a list of the best writing advice I've been given (that I can remember). In no particular order:
Don't chase trends (but be aware of them)
If something you're working on happens to fit into a current trend, then play that up and take advantage of the situation. If a current trend really sparks your imagination and fires you up to write something like it, go for it. But if you try to write something just because it's currently hot, you probably won't do a very good job of it, you'll probably be miserable, and by the time you get a book written and submitted, the trend may already be on the downswing. You're better off sticking to the stories you're passionate about and doing them really, really well. Then maybe you'll set the next trend.
When you're stuck, make a list
This was a lifechanging piece of advice for me. If you don't know what to do next or if you've got a scene with not enough going on, make a list of at least twenty things that could happen. That will force you to really think and be creative. You'll probably come up with a brilliant twist down in the last few items, or else you'll find the element that will give your scene a spark. The trick is to force yourself to keep thinking even after you've come up what you think is the perfect solution.
If a scene isn't working, look for the goal and conflict
Each scene should be about someone trying to accomplish something, with something else trying to stop that from happening. If no character in the scene has a clearly defined goal, then the scene isn't really necessary. If nothing is getting in the way of the character achieving the goal, then you can probably sum up any important action in a sentence and move on to the part where there's conflict.
A touch of the unexpected makes for interesting characters
Each character should have at least one element that doesn't seem to fit. Not necessarily something out of the blue, but something unexpected that only starts to make sense once you really get to know the character. There's something about that surprise or that "something here doesn't fit" that makes readers latch onto characters. The tough guy who turns to mush in the presence of a baby is a little more interesting than your standard-issue tough guy who doesn't turn to mush for anything, for instance.
Let it rest
It's really hard to be an objective judge of your own work, particularly when it's fresh. Either you'll hate it more than it deserves because you only see the flaws or you love it more than it deserves because it's your baby. It's very easy to see what you meant to be there rather than what's really on the page. I've found that I do a much better job at revisions when I put a project aside and work on something else for a while before I look at it again. Then I can see what's really there and fix it.
Finish the book
In most cases, you're better off finishing a project instead of flitting from project to project. Once you're a more experienced writer, you can judge when a book isn't working and move on to something that will work, or you can sell based on a partial and only worry about writing the book when you're under contract. But when you're starting out, finish the book, even if you hate it and have a dozen better ideas. Even if the book remains hidden and never sees the light of day, you'll learn a lot from getting to "the end." Your next book will be much better, and those dozen better ideas will have had more time to process in your subconscious, so they'll be more fully formed by the time you're ready to use them.