Another easy way: Turn off the ignition, remove the keys, open the door, lock with the central locking button, place your keys on the seat as you step out, close the door.
EDITED: Oops, my bad, this can't be done. Drivers's door lock is inhibited with driver door open. Modified way (passenger needed for this): Turn off the ignition, remove the keys, place keys in console, open the door, get out and close the door, passenger open the door, press central lock, get out, close door.
I think the noodle was talking about with the keys in the ignition....although check this out: With keys in the ignition, rear seats are folded down, pop trunk with remote or remote latch release, lock with central button, crawl out the trunk, close the trunk.