Focus A wiki needs to be about something to be successful. It needs to appeal to a niche that isn't entirely satisfied elsewhere and its main purpose is to share information. For example, some of the typical goals for a technical wiki are to help the community solve problems or to share similar experiences with a product. Information types that support these requirements can include FAQs, troubleshooting tips, examples, customer best practices, cheat sheets, etc. That said, wikis can be used for any type of purpose or audience, so they can consequently vary greatly between focus and structure. The important rule of thumb is to establish and understand the goals for your wiki early on and implement it accordingly.
Start with structure Wikis are organized in a bottom up manner. The content that goes in the wiki has a need to be shared. Therefore the structure does tend to define itself over time. Once your wiki has enough content, you can look at what is being posted and let that drive a more formal structure.
Plant the seeds It's important to seed a wiki to get the community started. Start with a loose structure and modify content over time to define how the wiki is organized. Encourage contributors to add their own content areas and determine how to tie the content together structurally once you understand what the community is talking about.
Calling all users Post links (with a description of why someone would be interested) on forums and blogs after you have some starting structure. This is where most of the traffic comes from followed by email referrals. Forum moderators can also entice users to document their answers or solutions in a corresponding wiki.
R-e-s-p-e-c-t Reputation is important to community users. As with blogs and forums, it helps contributions if the biggest contributors on a page are recognized.
Regular reviews Make sure the wiki content is current and correct. Although this should be done by the wiki community, it's important to have a small group of people (experts), including the moderator, who does this on a regular basis. A wiki with updated and correct information will continue to grow and prosper. When the site is reviewed for content, it is also a good time to look at the wiki structure and ensure the most interesting and useful content is easy to access
Content is king Adding useful content to a wiki should always be one of the main purposes of a wiki, especially a wiki dealing with technical information. Users should be encouraged to add information without worrying about making it perfect or polished. Don't consider a wiki to be the next great novel or a technical writing masterpiece. Remind users that a wiki is a community effort, so there will always be others out there who can help make the information more clear if needed. The bottom line is to help others solve a problem or better understand an issue. If your information does that, then you've succeeded.