What can the average person do to their car to make it more aerodynamic?

One simple thing you can do it keep the windows closed when you are moving.

You can't cheat the wind, but you can make sure you always close all the windows and use the air conditioner if necessary. An open window will definitely disturb the airflow, and reduce the fuel economy in all vehicles, but particularly as the speed goes up, since the drag coefficient of any vehicle increases at the square of it's speed.

Front spoiler or airdam which will lower the amount of air going under the car. The chassis is the least aerodynamic part of the vehicle and reducing the airflow there might give you some noticeable results. You cant do much about the rest, keep the car clean and waxed.

You could look for a body kit, maybe a slight lowering package - anything to round off the body and make it smoother.