What are good ways to encourage people to contribute knowledge to enterprise wiki systems?

You need to show value and you need to show how it works.

There are three important parts to that sentence: (1) you, (2) value, and (3) show.

1. Someone has to take the lead, and looks like you just volunteered! In a company setting, someone needs to lead everyone to the wiki.
2. There has to be a compelling reason to use it, even if that initially just means reading and researching content. Actually, its okay if most people are only reading right now - in an enterprise setting people are unlikely to voluntarily contribute if they aren’t readers first.
3. And that gets us to “show”. It’s not sufficient to tell people about it or how to use it or why it’s great. They have to see to believe. Seed it with content. Walk people through how to use it (especially important if you have non-technical users). And adapt daily processes to use the wiki.

The specifics of the last part depend on what your company does and its culture.

• Pick one process to start with and figure out how to simplify it for everyone involved using the wiki.
• Don’t answer knowledge oriented questions directly over email. Update the wiki appropriately then point.
• Have leadership interact with the company via the wiki.

One of the biggest obstacles as you note is that “people are already busy with their jobs”. If collaborative resources like a wiki require additional work on top of people’s jobs, those resources will rot. Period. Make them part of people’s jobs, make them make people more effective at their jobs, and show everyone how it’s done.