A simple and reasonable step would be to require the contributors to acknowledge that they own the copyright to anything they post, as a required part of your posting procedure -- A checkbox on the submission form that must be ticked, for example, in response to a clear declaration of authorship/ownership.
This won't necessarily protect you against litigation, but it will show that you *did* make an attempt to prevent copyrighted material from being posted. As such, you're more likely to be treated as a third party to copyright violation, instead of being partly-responsible for that copyright violation.
Also, be sure that you as the Webmaster are easy to get in touch with, so that copyright violations can be easily reported to you for action. I have had the situation come up before where the Webmaster had hidden behind private domain registration and had provided no on-site contact information. I looked for 20 minutes, and then sent e-mails to "webamster@", "postmaster@", and "admin@", hoping that one or more of these "customary" e-mail addresses would work. Seven days later, having received no reply, I gave up and submitted the DMCA claim to have the offending pages removed from the site and from search...
However, eventus has it right -- Consult with an attorney if you need legal advice.