Write Like You Mean It
I once edited a manuscript that was both the best and worst thing I had ever read in my entire career. I knew the author personally, and he was a natural communicator--funny and direct--but his book was giving me headaches. Although he made excellent points in each chapter, they were buried in a swamp of cumbersome, obfuscated language that often required several re-reads per sentence just to make it to the next amazing insight. The whole experience was like sifting for diamonds in a mud pit.
Finally, I asked him why on earth he was writing this way, so different from the way he spoke. His answer, somewhat dogfaced, was that he'd submitted the manuscript to various publishers without success, but the biggest one had written him back with this advice: his material was complex and would likely only ever be read by scholars and professionals who were used to academia, so he needed to "elevate his language."
It was the worst writing advice I've ever heard, and it's given out all the time.
Writing is communication, pure and simple. Good writing is good communication; bad writing will communicate ideas poorly. When you write in the same language you use while speaking, your words will flow with conviction and authenticity. Try to sound like someone else, and your words will sound the same--cheap copies of somebody else's concepts.
Reject embarrassment, and put aside any self-consciousness or insecurities. Start writing, and don't stop to think about how anything "should" be phrased. If you find yourself imitating phrases or styles you've read elsewhere, stop. Keep asking yourself, "How would I say this?" Better yet, speak the lines aloud. If they sound natural, you're solid. If anything sounds rehearsed, scrap it and start over. Keep at it until you have what you need, until it sounds like you, until it's real.
Your voice is too important to disguise.