Lipski's recommendations are certainly valuable but I suggest we step back a bit. Start with an organizational style guide. This guide is for grammar and writing style. The grammar/writing style guide supplements an industry tome, such as the Chicago or AP guides.
When that is done, create a design guide. CrackerJack Marketer, for example, suggests:
- Logo Guide – Guide for size, colorization, and proper placement.
- Color Scheme – Specific primary and secondary colors and combinations.
- Font Guide – What typestyles to use, and when they’re appropriate.
Need a place to start? Take a look at Richard Nordquist's style guide (reference work).