Try clenching your teeth together, and every time you feel them not in 'contact' anymore, you know you might be opening your mouth again so clench them lightly right back. I don't have a problem with this, but I do have a habit of making 'the' most weirdest and funniest faces while riding and jumping. In fact, it's quite amusing! =P Hope this helps.

That's so funny!  When I was a kid my mum used to moan about me 'catching flies' and I did just that.  I was out on a hack and obviously had my mouth open as the next thing I knew I was coughing and spluttering because a fly had flown straight to the back of my throat.  Yuck it was horrible!  I'd forgotten all about it til I read your post.