Rule #1: Hold On Loosely
In the proprietary production world, what matters about a copyright is who owns it. In the free production world, however, who owns a copyright is relatively unimportant. What matters is what license it is offered under. There is a very simple rule of thumb about the best license to use: use a “free, copyleft license”.
As usual, Terry Hancock forensically analayses issues of copyleft, the GPL and licences generally and you can read the full article at Freesoftware Magazine.