Finding Collaborators for Long Term Projects that Span Years and Maintaining a Good Relationship
If the collaborators are separated geographically, meeting regularly in person will keep your project vital, provided the visiting arrangements are conducive. I've used Research in Pairs (it can be more than two) at Oberwolfach, Research in Teams at Banff, and this coming summer will use Research in Paris at Institut Henri Poincare. These programs provide every inducement and aid to production, and absolutely no hindrances. They are invaluable.
The essence of successful collaboration is mutual respect. Good ideas should be acknowledged explicitly, criticisms handled with great tact. Electronic messages of any import are best aged overnight, and reread before sending. This is particularly important when the collaborator is not extremely well known to you, since in that case electronic messages do not accurately convey emotional tone.
In my observation, credit issues are the landmines of collaboration. The longer and closer a collaboration, the harder to manage this issue may become. As soon as a credit issue is voiced explicitly, pay close attention and seek a mutually acceptable resolution. Just as importantly, be alert to unexpressed credit resentments that may be simmering in your collaborator or yourself.