Visualize Something Boring
We like this one in particular. The beauty of it is you can turn a negative into a positive. Just visualize that you are someplace that you have always found extremely boring. It could be listening to a particular teacher who was so boring that he or she almost always put you to sleep. Perhaps it's some friend or acquaintance whose incessant talk and theories put you to sleep. Maybe it's your work, maybe it's your commute each day.
Whatever it is, visualize it. And recapture that bored, tired, heavy, sleepy feeling that you always experience. Let that feeling spread through your mind and all through your body till you're filled with complete tiredness and sleepiness. It works.