While starting to work on your game idea and especially during the first stages of the development, the process of formalizing your thoughts can help you to better conceptualize & design. As well written rules should answer at questions that any player may have, the process of completing your rules should lead you to ask yourself a lot of key questions about your game idea. Every time you solve a problem that arise, you improve your game design. In the iterative process of game designing, writing the rules is a key step.

The rules should be complete and consistant. You shouldn't being able to add or remove any sentence without changing the overall meaning. You must be driven by the seek of clarity, brevity and completeness. The first draft and the working text of the rules shouldn't include any external elements (humor, background description or story line...). It's a clear description of how to play your game, not less but not more than that!

Writing the rules is a iterative process on his own. Each version should be tested (with several playtesters), analyzed and refined until you're not fully confident in their fundamental qualities.

Graphics, diagrams and examples of player's actions are really useful to ease the reading of the rules and to help people to quicker understand them. Always prefer a clear graphic than a long textual explanation.
People generally don't like to read a new rulebook, so anything that could ease the reading will be appreciate.
For a video games, there are explicits and implicits (under the control of the software) rules. The core rulebook becomes a communication tool with the team of developers. Every things that are needed to define how the game system works, should be provide in the rules.

If your game is quite complex and require more than just rules, you may consider some alternatives. Many people don't want to read rules to play at a new game so try to innovate in order to overcome this hurdle. For examples you can provide:
- Illustrated rules (like a comic or a manga)
- Summary Sheet (Turn Phases, Scoring System,...)
- Example of 2-3 turns, players follow the example step by step and discover while playing the rules.
- Flash animated tutorial
- Digital prototype that you can play alone
- Gamemat
You may add any relevant supports that can help people to try and play your game without reading the rules.

A useful ressource from Da Vinci (Italian Publisher): How to write a standard rulebook.