There are millions of variations but some that stand out are;
Banachek's one from Pre-Thoughts which is essentially is what chair will be left empty.
Spelmann has a nice one on the Chapters of Marc Spelmann that requires a little more of a set up but is quite nice. I personally think it is a little over kill however lots of people that I know love the routine.
Spelmann also has a nice one in his lecture notes 101108 which I cannot recommend highly enough.