Brad Frost has some solid advice in his Style Guide Best Practices post.
The benefits of style guides are many: they establish a common language, make testing easier, save time and effort, and create a useful reference to keep coming back to. And most importantly, it lays a future-friendly foundation for your organization to modify, extend, and evolve over time.
Each point he makes is important, but there are two that seem to get overlooked in many style guide projects: Make it Cross Disciplinary and Make it Last. I’ve seen too many style guides that were directed at one, narrow audience. This means that other groups involved with the product are forced to make their own guide. The chances of these multiple guides matching up 100% are pretty much nil. At the same time, many style guides are not built for maintainability, which is important as a product changes over time.