One idea that no one has really touched on is actually not having the civies just wandering. You can give them a goal, destination, a purpose. I don''t know exactly how you''re implementing the civilians, but if they are of different character types, like baker, butcher, fisherman, etc, you could give them things to do. For example, the baker would need flour, so he may walk to the mill, then to a farmer to get milk, etc. They could also stop and converse with other civies they encountered. I know this would mean a fair bit more work, and probably not what you wanted to tackle, but it would definitely create a living world.
"There are nothing on the Earth, my friend Horatio..."
That idea is already implemented in many games, the earliest I remember is Commodore 64''s "Sherlock Holms", on PC I remember old an perfectly good quest "Lure", the last game using same "virtual life" I met is "Shanmue".
My opinion is that an illusion of a life is a powerful instrument of RPG designers to make atmosphere of the game more real.