There's not much writing advice that I put much stock in, but one piece of advice that I do is this one: know your characters.
I recently read a book that I loved. One of the revelations the book had to offer was that the two main characters had been engaged in a sexual relationship -- even though they were adopted brother and sister. Once I turned off my immediate "ick!" reaction (there's something about two people who were raised together -- even if they aren't biologically related -- having sex that immediately stirs that reaction), I realized it made sense for the two characters, and that it also made sense, given the character's personalities and the first-person narration, that it wouldn't have been revealed earlier in the series. (This was book two of a three-book series.) Reading the comments on the author's blog regarding that relationship, I realized something: she (the author) didn't seem entirely comfortable with it, either, or at least hadn't been when the story started to go in that direction. But she took it there because that's what those characters would do, and because it made sense within the story -- knowing full well that it could make people uncomfortable.
It worked well. At the end of the story, I was still rooting for those characters, still hoping for everything to work out for them.
I've discovered that most of the authors I really, really admire can tell you just about everything about their characters, often right down to what breakfast cereals they prefer and what kinds of pajamas they would wear. It's not a "genre fiction versus literary fiction" thing, either: contrary to the conceptualization that genre characters are not as well developed as literary fiction characters, I've found genre fiction pieces where the characters are incredibly complex and well-developed. I've also found literary pieces where the characters seem flat and cliched. (And vice versa, of course.) To me, fiction -- whether high-action and plot-heavy, or subtler and more contemplative -- is driven by its characters. I'll often give stories with lackluster plots a chance if they have engaging characters. Likewise, I'll give up on fiction that has interesting stories if the characters aren't interesting. Good characters are often more memorable than the particular twists and turns of the plot. If we're talking series fiction, strong characters are essential: if I care about what happens to the characters, I'll come back for more.
A lot of writing how-to guides that I've seen suggest things like creating character dossiers or answering pointed questions about your characters. (If your character was a tree, what type of tree would he/she be?) For better or worse, I started writing long before I realized how-to guides were even an option (most seven year olds don't spend a lot of time in the self-help section). At any rate, I'm too set in my ways, writing-wise, to follow such guides. Maybe they work for some people. They tend to make me feel like an idiot. Whatever. Not like I'm in any sort of position to offer writing advice to anyone.
For me, the getting to know my characters comes from the act of writing the story. When I started my current work in progress, I had one scene: a young woman wakes up next to a dead man; she knows she killed him, because this has happened before, but she doesn't know why. At the time, I didn't know why. I also didn't know what triggered the blackouts she went into when this occurred. Hell, I didn't even know her name. Some of these questions were answered very easily, but some of them took 300 pages of writing to figure out.
I've been hanging out with Dale (which is my protagonist's name, but not her real one) for a year now. I can tell you what breakfast cereal she prefers (Cheerios) and what she wears to bed (t-shirts, often with retro cartoon characters on them, paired with sweats in the winter or boxers in the summer). She reads a lot of children's books and fantasy, light and escapist. She watches a lot of old movies, and listens to a lot of old music. She picked out her current alias -- Dale Highland -- because the last movie she had seen was Flash Gordon, and the last book she had read was Outlander. She doesn't believe in using aliases like "Jane Doe" or "Jennifer Smith," because she thinks they're too obvious. She hates wearing makeup, but she'll do it because she often has to change what she looks like. She's not evil or crazy, but she thinks she might be.
I'm close. It kind of hit me last night. I'm working on cleaning up the climactic scene, and then there's one more scene I want to go back and do some major tweaks on -- mostly adding some stuff, because it wasn't quite as sensory as I wanted it to be. (In other words, it needed more heat.) Then a final read-through of the whole thing for consistency and continuity. And then...and then I'm going to take a deep breath and start submitting.
After that, I probably won't talk about this project much anymore, unless there's news. I don't want to jinx anything.
I've heard that writing for publication is not for the faint of heart. I'm trying not to be.