step 1: getting the Marvin's attention.
step 2: like step one except he is facing you
step 3: He rises in a vaguely zombiesque pose, front legs hanging down
step 4: Fully extended Marvin begins to wave his arms slowly, stretching them up towards you. The key to maintaining an airborne Marvin is to make contact before he sees you've noticed and lowers himself. If you begin petting while he's still upright he maintains it as long as gravity allows.
A more accurate (if freaky) portrayal of the fully upright Marvin.