I've found that meeting people in person breaks down a lot of the barriers. There's a guy that I work with in Germany who I thought was a jerk for many years. Once I finally met him, we get along great.

You do lunch with them, you ask their opinions about things, and don't pretend to have all the answers. Instead, you ask them about the best way to test their software. They'll buy into the process and realize you're on their side.