Well, mine is usually due to a anxious thought. So when this happens I try and focus on anything around me and just not let it go till that is the only thought in my head. I will also try and have a song that I will replay in my head to take the place of the thoughts. Another thing I do is wear a rubber band on my wrist and when I start to drift off I will just snap it. It usually brings me back to reality. Other then that I'm not sure. Sometimes it is very hard for me snap back and I will just go with it and make the best of it. If you have music available at the time I would try listening to that, pref something upbeat.