As you begin to write, make sure you are keeping your audience in mind. The more you do this at the beginning of your project, the easier it will be down the road to make sure your tone remains consistent. If you feel like getting into the numbers of it, take a few minutes to review my post a few weeks back about Writing in Plain English. Regardless of your comfort level with numbers and analytics, this handy tool can help give you a rough idea how understandable your writing is for your intended audience.
One more test that may work even better to judge the tone of your writing is to find someone in your target audience (and whose judgment you can trust) and simply give them the piece to read. They should be able to clearly understand the writing but also not feel like they are reading well below their comprehension level.
Find the perfect voice and tone for your writing and stick with it. As long as you maintain your consistency, people will begin to trust what you have to say.