grow your wiki
Any grassroots, or bottom-up, strategy is the best place to start since the success of a wiki depends on building active, sustainable participation and this only happens when people see that the software is simple enough to immediately be useful, and meets their needs without requiring them to spend lots of extra time.
A good first step is to identify a group or department who would likely benefit the most from using a wiki, and whose people are open to trying new tools. If you're looking to expand wiki use in another group, look for the thought leader in the group - someone who is very forward thinking, respected by peers, and willing to Champion a new idea and get others around them involved.
If you're the thought leader and wiki champion, congratulations! Encouraging others to use a wiki can be as easy as the wiki itself if you encourage the right patterns of behavior and content creation. Read on for more ideas, and feel free to add your ideas and strategies to this wiki as well!