Profitable Collaborations

What are the essential elements of profitable collaborations when creative businesses work together?

Some ideas about effective business collaborations in the creative industries are in these articles I've written:

 - Creative Collaborations and other essential C-words published by Creative Choices (2009)

 - Creating 'Interprises' published on the T-Shirts and Suits blog (2007)

I'm interested in your experience and looking for case studies about creative businesses working together. 
 - What were the challenges?
 - What were the solutions?
 - What's your advice to other creative enterprises considering collaborations?

Please tell us your own ideas, experiences and advice about profitable creative collaborations on the T-Shirts and Suits Creative Enterprise Network.

---