In the case I described, I was already laying down and simply stayed there. In general, your body position does not really matter. Well posture does, whether you're meditating or not.

Mindful breathing is great in general, but for getting rid of clenching and tension in my neck, I get the most mileage out of putting my awareness there and then either willing the area to relax or imagining that it is relaxing. I tend to alternate between the two.

I've also been trying to be mindful during the day to "check and release" any tension anywhere, but especially in the TOS related areas.

Even now I still clench my neck and traps and have to remind myself to release them. It's strange.