• Keep calls to action short and to the point.

• Use active verbs: "Contribute, Add, Build, Create" etc.

• Encourage conversation around a topic by pointing readers to the "Comments" section. (Even better, create a link to it so people can add a comment with just one click.)
  

• Try putting calls to action at the top or middle of the page, not always at the bottom (some people may not read that far!)

• Keep the formatting for calls to action consistent throughout your wiki--users will hopefully start to recognize them as "action points" on every page. (on wikiFido we use a graphic paw, but consistent formatting with a bullet, for example, works just as well.)
  

• Apply the same rules for style and formatting to your calls to action (avoid all caps, large fonts, multiple formatting styles, etc).

Here is an example: