|MadSci Network: Earth Sciences|
The most effective way is to show them something really big, and then show them the fraction that corresponds to the amount of time we've been around. Take them to New York City, show them the World Trade Center, and mention that if this is the age of the Earth, man's time on the planet is the top however-many inches of the building. Take them out on the school's track and make them walk around it a few times until they're all exhausted, then point out the few inches or so that correspond to "modern times". You can also use this to demonstrate how much money Michael Jordan makes in a year compared to what they will make in their combined lifetimes.
Try the links in the MadSci Library for more information on Earth Sciences.