It is the long history of humankind (and animal kind, too)
that those who learned to collaborate and improvise
most effectively have prevailed.
Charles Darwin, English Naturalist, (1809 1882)
Though you may have your own organization fully under control, it is the ability to partner outside your organization that is going to bring far-reaching success. A few things that I have learned in this area:
People are the Most Important Thing
The ability to work effectively with many different types of people is the most critical factor in the success of nearly any effort. Even in 'pure technology' it's the people on which the effort depends.
Organizations Also Need Interfaces
If you want more partners to connect then simplify the process. Think of the experiences enabled by the standardized USB cable then apply that principle to your organizational interfaces.
Partners are Better than Vendors
Whatever the official relationship might be, treat a vendor like partner and they will start to act like one. No matter what they are selling, they have knowledge and insight that is valuable to you.
Everyone Shares Some Common Ground
It may take time to find, but everyone shares common goals, beliefs or experiences. Identify those, then frame what you're trying to achieve accordingly and your message will resonate.