Everybody has a story to tell and, when writing for a client, the key to telling that story effectively is to essentially get out of the way and let them tell it. As the writer, it is crucial that you research the client's business/organization as thoroughly as possible and also work closely with them - and any other available resources - to determine exactly who their target audience is and what message will best drive interest and traffic. Also, what is the desired call to action? Figure out what this is and how you can deliver it.
This is all part of the prep work, and by identifying who you are attempting to reach, you can also conclude factors such as appropriate story length, tone, design, etc. However, when it's time to actually write the story, give the client as much voice as possible.
Perhaps this seems like common sense, but far too often writers attempt to overwrite, or put too much emphasis on their own voice. There are certainly mediums where that style can and does work, but in advertorial writing, the goal is to connect the client (business/organization) with the consumer. That is accomplished by allowing the two parties to "speak" with one another as directly as possible. Do not overcomplicate the process.
When writing the story, get as much information and quotes as possible from the client and make THAT the story, using your own words merely as segues, connecting one thought and point to the next and shaping the message in a way that it quickly and clearly reaches - and impacts - the target audience. Of course it will be necessary to edit the client's words and/or assist them in choosing the right wording, but it should be their thoughts and message, and that should be obvious to the reader at all times.
This technique is the best way to quickly establish a relationship between the client and their audience. This is the client's story to tell and they know what they want to say better than the writer does. This is, after all, their business. They are the experts. Just learn your client's goal(s), ask them the questions that will convey that message, and help make the story flow smoothly for the reader.