Planning for Usability is key for designing documentation for your readers. Depending on whether I am writing an email to communicate a idea or writing documentation for new a application being implemented, the approach is slightly different.
When emailing a group regarding a upcoming change, both the subject and the first sentence or most paragraph contain my main point. I then follow with details of the change and timelines. Each detail mentions whom will be affected, how and when.
Writing Documentation starts with an overview of the product. The overview typically includes what the product does, new enhancement and if it's an upgrade or new features. The middle of the documentation contains graphics and screen shots. I close the documentation out with contact information and document revision numbers. Because I have identified who my audience is, I am better able to determine what each reader needs to complete there individual task.