When I was talking about how to use feedback from others, I mentioned that you should consider whether any suggestions would alter your voice. Voice is a pretty tricky topic, so I thought I should address it. There's no real "how to" for dealing with or developing voice, and yet it's probably the most critical factor to an author's success. It's the thing that sets a book or an author apart from the crowd. When you look at what agents and editors say about books, they'll mention that the voice just grabbed them. A powerful voice can overcome all kinds of flaws in a book. And yet it's not something you have a lot of control over.
What is "voice"? I think it boils down to the things you say and how you say them. I've been to writing workshops where the presenter will read excerpts from books and ask the audience to identify them. The idea is that these authors all have such strong voices that you know right away who the author is. That may be true, to some extent. There are little touches that come through, no matter what an author writes, but I think there's a problem if an author has the same voice in every book. I've been writing a contemporary fantasy series in which my first-person narrator is a rather sarcastic young woman from a small town in Texas. I just sold a young adult steampunk fantasy novel in which my narrator is a rather sheltered and naive, but highly educated, teenage girl in a Victorian-like society. These books had better have very different voices because the books and narrators are so different. I find that when I read aloud from these books, it's not just my accent that changes. Even the speaking voice I use is different. But I think both books are still distinctly "me." I'm not sure that they could be used as an example in the kind of workshop I mentioned, where you could read an excerpt and even tell that they're by the same author, but I do think there's something in them that means people who like the one series may like the other.
So, if the voice of each book needs to be appropriate to that book but you still need a distinctive author voice, what does that even mean? I still think it comes back to the things you say and how you say them. Think about the entertainment you enjoy -- books, movies, television. Make a list of your favorite characters and the things you like about them. Do you see any patterns? What about stories or plot lines? Is there any plot element if you see it mentioned in a show or movie description or on a book cover, you're instantly hooked? Those are the things that should make it into your own work. If you write about things that make you excited and passionate, your enthusiasm will come through and create a sense of voice. Writing about things you love means your work will truly come from the core of your being.
When it comes to the way you say things, it may vary by book, but there are still likely some commonalities. I know that my writing style is an odd mix of terse and to the point and almost baroque in sentence complexity. That probably comes from a childhood and youth spent reading Victorian novels and epic fantasy and my training as a broadcast journalist. I love crossword puzzles and finding new words that strike me as vividly descriptive. And yet I'm not big on physical description. I have very clear images in my head, and I can get very clear mental images out of even the tersest writing, so I guess I just assume everyone does that and doesn't need lots of words to tell them what to imagine. Dialogue is where I like to play. Those things are probably common to everything I write.
I had a couple of moments that I think were instrumental to me developing my personal writing voice. One was in college when I had to write a lot of papers for a class. I'd been making reasonable marks on them, but not as good as I hoped, and I was trying perhaps way too hard to write in the way I thought you were supposed to write a paper. Then near the end of the semester, I was getting frustrated with never being quite good enough, I was writing about something I found fascinating, and I had a pretty tight deadline. Instead of slaving over trying to write a "proper" college paper, I just wrote what I wanted to say. That paper came back with the highest grade I'd had in the class so far. It seemed I'd found my voice, and being myself instead of trying to be what I thought they expected paid off.
Though I suppose I still hadn't learned my lesson because a few years later when I was starting my career as a novelist, I was writing category romances. I'd sold a couple to a smaller press, but all that time I'd been very carefully writing what I thought that kind of book should be. On the third book, I guess I was more confident because I just wrote, and that was the book that became my stepping stone to a larger publisher. Something clicked, and I knew that book was more "me." Since then, I've learned that whenever I try to be a certain way or give them a particular thing that requires me to do something that feels unnatural, it's not my voice. Your unique voice should just flow naturally from you.
About the only voice-finding exercise I can think of is to do just that -- forget about rules and guidelines or expectations, the way you "should" be writing, and just write the way that feels right for you. Then take a look at it. It may still need editing and fine-tuning, but the way you write when you think no one else will read it may be the purest form of your voice, and will likely have more life to it than when you're working hard to be what you think is expected of you. It may be more challenging to learn to adapt that to different characters and different kinds of books, but you have to find your personal voice to even start.