I don't know if this is an option for you, but my husband put in a second door, about a yard or so back from the front door, so we have a vestibule, and only open one door after making sure the other is shut. It was easy in our house, because the front door opens onto a narrow hallway. A friend with several cats liked it so much that she had a vestibule made out of plasterboard (her front door opened directly into the living room). Her parents, dog owners, did the same thing, but used glass blocks for the side walls to keep it bright.
Another idea is a motion-triggered alarm installed inside the door at "cat level". I've never tried it, but have heard that it works. I've seen battery-operated ones in the form of little animal "statues" in home improvement centers here. They're meant for outside, often to keep cats from using vegetable patches as litter boxes, and stop "wailing" as soon as the cats distance themselves. One pet store chain is now advertising them as "cat door alarms", and I know from another cat site that they do work with most cats.
P.S. Somebody here - I think it was yayi - used a motion detector to keep her cats away from the Christmas tree. If I remember correctly, it played a Christmas carol when it was set off.