How do you get members to contribute their time, knowledge or skills for free? Many business models rely upon it.
You can’t. People don’t work for free. Ever. But we do work for things other than money. We work for love, appreciation, power, fame and a dozen other things.
Your challenge isn’t to persuade members to work for free, but instead to pay them in a currency they value more than money.
Try this. Assume that your members want to feel important. This was the key insight of Dale Carnegie's How to Win Friends & Influence People (make sure you've read it).
If you want members to work for free, you may try to pay them in importance. You might let it be known you consult the top members for advice or give best members their own custom URL for their profile. Perhaps you might consider giving admin access to areas of your site or awards to the top member of the month.
The message is if you want people to work for free, you have to ensure you pay them well and let it be clear to the entire community of the benefits from contributions.