A long time ago, I went to the Tacoma Chapter RWA's Manresa Castle conference, and there I was one of two people who attended the "poetry in prose" workshop given by a local poet. I sincerely pity the people who didn't attend. There, I learned what voice was, and how to identify it, particularly my own. It's such a difficult thing to identify your own voice. I wasn't sure until I attended this workshop, and that's after writing nine novels. This workshop literally changed my writing life. It gave me one huge tool to work with and sparked a lot of realizations about the interaction of voice and story structure. It also gave me a lot more confidence than I had had about my writing, because I found my voice was already there in my books.
I don't think you cultivate your voice so much as discover it, although once you do understand what it is, you can work to make it stronger and more effective. As beginning writers, you naturally copy the voice and style of those authors you like best--that's the learning process. But your voice is made up of all the influences of your life: the books you've read, the part of the country in which you live, the dialect or language you speak and its rhythms, the music you listen to. Think about all the sounds you've grown up with. These things will influence your voice.
Think of the images in your life that speak to you most. These also will influence your voice, because that's the source from which you take your metaphors and similes.
Voice is your natural sense of poetry--your own and others'. This is the reason why you're clumsily told, "don't use adjectives or adverbs." You're rarely told why, and what effect this has on your work. Of course, you can and should use adjectives and adverbs when you write, but not exclusively. If you do, then you've stripped your story of your voice. It's why I've instinctively railed against people trying to eliminate "wandering body parts" sentences entirely--it takes away yet another poetic device (idioms, metaphors, and similes) that aids one's voice.
Voice is also tied to the emotional pitch of your writing. That is, when you are writing a particularly emotional or vivid scene, that's where your voice appears the strongest. It's not something you can or even should maintain in every paragraph of your story, but it's something that rises and falls with the pacing and structure of the story. When your voice ebbs and flows in sync with the structure, it contributes to narrative drive--that is, it's what keeps the reader turning the pages, even if the action is mundane.
Here are two examples of the same idea; one stripped of metaphors, similes, and most of the adverbs and adjectives (and thus, voice), the other as it was written:
"She sat in the drawing room at the pianoforte but did not play any music. She looked out the window at the sky instead. She didn't want to be here, and in fact felt unhappy and trapped. She wished she could go away."
This paragraph is short and to the point, and that's fine. Sometimes you want short and to the point. However, it doesn't have as much emotion as:
"She sat in the drawing room, her hands on the keys of the pianoforte, but did not play. Instead, she looked out of the window at the blue summer sky. The panes formed small rectangles, the sun casting shadows on each edge--it looked like a cage. But then a movement caught her eye: swallows dove toward the house, outlined in each frame of window like quick sketches, then with a quick flick of their wings twisted away from the house and upward into the air. She envied them; she wished she had wings so swift that a mere glance of a wingtip would send her away."
Here, I don't say directly that she feels trapped or that she's unhappy. But all the images combined--not playing music, looking out of windows that look like a cage, swallows flying away, and the repetition of the word "away," and "wing," etc.--give an impression of her emotional state. Note also that I use concrete images--I say "swallows" instead of just "birds." Read both paragraphs aloud, and listen for a rhythm. I think the latter paragraph is a bit more rhythmic.
This is voice. My particular combination of rhythm, concrete images, metaphors, and similes used together to evoke a scene and infuse it with emotion. You, no doubt, do it too.