Why is writing so important?
Do you think a developer only writes code? Communicating with clients/partners via email, giving updates in bug trackers and stories, writing the stories themselves, creating documentation, comments and help files... These are all responsibilities of a software developer. Even writing easy to understand code is communicating well!
A few initial tips
You 're not writing a novel
Don 't go overboard with the language used. Keep it as simple as possible but don 't shy away from technical terminology if that 's the correct level for the document.
Know your audience
Write at the correct level of detail and with the correct terminology for your audience. Don 't write development details on a user guide or marketing pitches on the API documentation.
Understand the problem before writing
Confused minds write confusing documents. Understand the problem and draft everything you want to include