It's the boring part of developing a new community: setting the Dos and Don'ts. While Community Guidelines will be an ever growing part of your community, getting some basics down will not only give users a clear idea of what's right and wrong, but it can also go a long way toward developing your community culture.
1. Accentuate the Positive.
Instead of focusing on the Don'ts, think about the Dos first. What kind of behavior do you want from your users? You want them to be nice to each other, you want them to refrain from four-letter words, you want them to help out other users and answer questions?
2. Check out the guidelines of the communities you like.
As well as some communities you may not like. Read through their guidelines. Note what you like about them, as well as what you don't like. It can even be helpful to try and determine what prompted a particular guideline and consider if that situation might occur within your community.
3. Consider your community as it is right now (or will be from the start).
What are the special concerns of your community? Instagram (http://help.instagram.com/477434105621119/) and Flickr (http://www.flickr.com/guidelines.gne), being photo sharing communities, put a big emphasis on making sure that the content you upload is your own. In contrast is Get Satisfaction (https://getsatisfaction.com/corp/help/community-guidelines/) who mentions content rights toward the end of their guidelines, but, as customer service community, they focus more on how to interact with other users.
4. And then think about how you'd like it to grow.
Adding in a few guidelines that might seem like overkill right now may come in handy if you find yourself scaling up quickly.
5. Talk like a person
6. Provide ways for the community to self-regulate.
If you can build in (or turn on) flagging functions or provide a venue for easy reporting, you community will let you know when you need to step in, freeing you up to grow the community instead of playing referee. Make sure to point to these in the community guidelines.
7. Don't overedit.
Think of the Community Guidelines as your site's Constitution. While the ability to add amendments is important (especially when major new functions are released, for instance), some things don't need to be handled at that level. Don't make a super-specific rule just because of one incident in your community. Look how well that worked with prohibition.
(Image: Law code of Hammurabi - King Hammurabi with Shamash, a Creative Commons Attribution (2.0) image from savingfutures's photostream, edited)