The best and most successful co-writes I have experienced were at first getting to know the other writer. I usually just talk with the other writer sharing experiences I have had and talking about friends family or anything else that come up. One of my most successful co- writing partners and I did not write a song for the first two sessions but on the third session wrote a song that has been on hold for several major artists but not cut yet. This gentleman and I have written several other good to great songs. Secondly dump your ego especially if you are writing with a more experienced successful writer. I have been lucky to have written with three number one writers and have learned a whole lot about craft just by keeping my mouth shut and listening.Also rewriting is invaluable I hope this helps.