Interview the subject matter experts (SMEs) to find out "the real story" behind the product. When they explain how the product works in technical terms, ask them to explain what that means for the consumer. The consumer buys technology to help them do something. Your manual needs to help them use their technology to do that something. They do not care about the advanced engineering behind it.
Explain the problem being solved. Many technical manuals spend pages and pages explaining every technical feature of their product, while neglecting to explain why a consumer would ever want to use those features. Spell out why the consumer should care about a particular feature. For example, this new and improved feature allows you to check your email while you are surfing the Internet --- and it is a lot easier to use than the old feature.
Write to help the consumer accomplish an activity. Make the explanations short and simple and include illustrations. Remember, people do not want to read manuals; they want to use the product to do their activity.
Write in a simple, conversational style. Explain the activity like you are talking to your best friend. Your tone should sound something like this: "This is a really great feature. Let me tell you why. Here's what you can do with it."
Try to create "aha!" moments for the user, as in, "Aha! That's how you do that!"
Edit your manual to remove overly verbose or technical sections. Check to make sure that your writing is clear, simple and easy to grasp. Run it by a friend or colleague who has little experience with the product. Ask him to do the required steps and if it is easy to follow.
Make it enjoyable to read. Keep your writing light and conversational and don't be afraid to add a little humor or a funny anecdote. The more "fun" your manual is to read, the more likely consumers will read it --- and the less likely they will be to call the product help line.