i do it too.
if i ever notice myself doing it, ive learnt something that helps - i press the tip of my tongue to the roof of my mouth, and let my lower jaw droop a little (obviously not in front of people, or else try and make it subtle).
this also helps if you get panic attacks, because it helps to stop you breathing in and out too much, so it helps get your breathing under control.
but i clench my jaws so often, and dont realise it half the time, so im getting lots of headaches at nite!