Stand feet shoulder length apart. make sure you have some room, at least three feet all around.
Breathe normally, deeply. You might start with a count.
Now comes the fun part:
With your right hand, connect moderately hard with the left side of your chest. Hard enough to knock some air out. At the same time, with your left hand, connect with the center of your back, a little higher than the ribs.
Reverse the motion, so that you're hitting your right side of your chest with your left hand, and the center of your back with your right hand.
Repeat, as quickly as possible.
One can also do this on thier knees, if that suits you better.
It is, in its own way, like flogging, but it leaves fewer nasty marks.
Do this long enough, and you'll definitely begin to trance out.