Writer's voice is one of those things that doesn't get a lot of play in fan fiction, just because a lot of fic writers, I think, tend to be relatively inexperienced writers. They're still working on the basics of plot and character and dialogue. But besides that, I think that there are misconceptions about voice, in that I often see people talking about it as if it's something they have to find and lug home, kicking and screaming and clawing at the furniture.
Every writer has a voice. It's the way you think, talk, and write naturally. You don't need to create it. You may need to practice it, or analyze it in order to hone it or change it, but it's already there. There's really no such thing as voiceless prose. What may seem a perfectly transparent piece of writing in 2009 would appear to have a distinct voice to a reader from 1909, or 3009. So the goal for a writer, I think, is not so much to have a particular kind of voice, but to perfect the voice you have (or the voice you want) whether it's translucent prose, or intense lyricism, or gritty realism, or arch humor.
If you want to polish your voice, it's a good idea to make sure that the underlying mechanics of your writing are in good working order first. Spelling, grammar, punctuation, vocabulary. All the boring stuff. Those are the components that make up your voice - what words you choose, and how you string them together. You can certainly work on voice when your grammar is shaky, but in a lot of ways it's like putting a fabulous coat of paint on a car with the engine rusted out. You will be able to express yourself better if you have a good grip on the basics - or a beta with a good grip on the basics.
Once you've got the basics down, step back and take a look at your own writing. Pick something you haven't read for awhile; the more distance you can get from it the better. Read it as if it were someone else's story. How are the sentences put together? Do you tend to use the same kinds of sentence structures a lot? Lots of dialog, or very little? Do you use certain words, or types of words, more than others? Lots of description, or hardly any?
You can experiment with different voices. This is my normal voice:
Pencil-thin shafts of sun expanded instantly into a blazing searchlight. Two, caught in the spotlight, burst into flame like a torch. William fell (in a fashion more reminiscent of one of the more dashingly Miltonian versions of Lucifer, he hoped, than a clumsy git who'd lost his grip on the conduit) driving into Two's face feet-first. The screaming monster hit the ground, and William rolled out of the shaft of light before he could catch fire himself. His exposed skin was crackling, and he could smell the burnt-bacon stench of charring flesh. Christ, it hurt, but he was alive - he'd gambled he could endure the light longer than these far-more-flammable creatures, and he'd won. One down.
Compare this with "The Sure Thing," a Damon Runyan pastiche, in which I'm deliberately imitating the style of another writer:
You look to me like a new kid in town, and on that account, I will give you some friendly advice. If you stick it out in Sunnydale long, you will see ins and you will see outs. The Mayor is in, the Mayor is out. The Master is in, the Master is out. Angelus is in, Angelus is out. I take it philosophical, you know? We have some unlikely residents in this burg, namely the Slayer, and that rotten traitor William the Bloody, occasionally known as Spike. At one time, Spike is in, and while he is in he paints the town a fetching red. But then he is out, and his doll gives him the heave-ho, and he falls for the Slayer, and it just goes to show that where dolls are involved, it can only go from bad to worse.
Or with this paragraph from a WIP in which I'm attempting to overlay a sort of 'fairy tale' style upon my own:
Once upon a time there was a girl named Constance, which means 'steadfast.' Her mother was a brave warrior who traveled the land slaying monsters, and her father was a monster who had resolved for love of her mother to give up his evil ways. She had two brothers and two sisters, and although she was not a princess, her father treated her like one, which was perhaps not always the wisest of courses - but her father was better known for his great heart than his great wisdom.
Take a story you like by another author and do the same thing. Try to find writers with different styles, and compare their voices - how does Herself's voice differ from Nan Dibble's? From anaross's? From lordshiva's? From Kita's or Anna S's? For bonus points, are there broad differences in voice between fanfic and published fiction? If you've read fanfiction and published fic by the same writer, how do they differ? How are they the same? If you're aiming for lyricism, pick some poetry apart.
None of this is going to give you your voice, remember - you already have it. Doing exercises like this will just allow you to understand and control it better. For example, my natural inclination, my natural voice, is to use a buttload of dashes and semi-colons. I don't want to get rid of them entirely; I like them. But what I do want is to make certain that I'm using them effectively and gracefully. If see that I've written three sentences in a row with the same basic structure, as below:
Buffy didn't hesitate - she leaped into the crevice, sword drawn. Spike followed her, and a moment later motioned for silence - his keen ears had caught the drumming of footsteps still too faint for her senses. She halted and pressed close to the sweating brick of the sewer tunnel - could they be far behind their quarry?
That is way too damn many dashes. If I'm in control of my voice, I can go back over my first draft, decide which sentences need the dashes most, and which would benefit from being broken into shorter sentences or revised in some other way (semicolons, ellipses, plain ol' commas.) Like so:
Buffy didn't hesitate; she leaped into the crevice, sword drawn. Spike followed her, and a moment later motioned for silence - his keen ears had caught the drumming of footsteps still too faint for her senses. She halted and pressed close to the sweating brick of the sewer tunnel. Could they be far behind their quarry?
This preserves the voice of the paragraph, while eliminating excesses that could be really annoying to some readers. (I say some, because a lot of readers don't take any more notice of voice than many writers do.)
The bottom line is, voice is not something artificial that you impose upon a story from outside, or it shouldn't be. It's an organic outgrowth of the way you think and speak and write, polished up and made spiffy. Done correctly, voice enhances your story, it doesn't overwhelm or obscure it. It's not a matter of having to choose between voice and clarity of storytelling. If you learn to do it well, you can have both, and your story will be the better for it.