It's a familiar formula. I do something so that one or both of my trapeziuses spasm. I get it massaged. I take some Aleve. I take a hot bath. I go to bed (it's usually in the evening, after a build-up during the day). I try to get my neck as confortable as possible.
And then I wake up some hours later. It's dark. And my neck isn't hurting. That feeling of not hurting after hurting so badly is just amazing. It's probably not worth deliberatley hurting myself just so that I can feel the relief, but it's close.
It usually comes from sitting at the computer too long, or laying down reading in a bad position or something. Yesterday was the first time that it's happened from carrying a large object. Luckily, it didn't start really hurting until a few hours later, when I was closer to being able to do something about it.