Relaxing neck and shoulders

I have just caught myself at it again - shoulders totally tensed and practically up around ears, neck constricted, back tense, perched on the front of the chair. I have forced myself to relax it all but I know in 5 minutes I'll be all scrunched up again, every muscle tense, like an animal waiting to pounce.

Has anyone got any tips on how to stop this habit? I get such awful persistent headaches because of it. I have nothing serious to worry about, so I don't know why I do it. There I go again - all tensed up in the time it took to type this.