In my experience a spec will have more chance of being read if it has the following:
- Use diagrams where possible - pictures are worth 1000 words
- Have a title page that clearly indicates what the spec is describing
- Have a style that is used throughout the document. Make all headers the same font, size and style. Make the font the same all the way through, use the same bullet styles etc
- DONT WAFFLE - Be clear concise and to the point, and don't add extra cruft to pad out your document. If a point can't be explained in a few lines of text, then maybe you need to break it down further
I have seen in companies where the person writing the spec doesn't understand the system. It's almost a way of learning the system by writing the spec. This usually ends in tears...