Keep it about 3 inches away from the wall, don't put it inside a cupboard/drawer/unit while playing it.
DEFINITELY keep it off the ground, as this is where the majority of dust gathers.
When i got the YLOD my PS3 was on it's side (standing up) AND on the floor, so I might suggest having it flat (across)
You won't be able to completely prevent dust from getting into the fans, but if you are really adamant that you want none whatsoever, you could clean the entire case / fans out when you get it , and do so every few months