Through timed exposures! Heh! There were a bunch of kids goofing around in the bottom of this picture at the time I snapped it off. They were mnoving around enough that they just didn't show up on the exposure. If you look in the lower left though you can see some of their friends ghosted there. There were stationary enough that my camera magic didn't make them go POOF!
This was a 6 second exposure.
The ever growing set is here: http://www.flickr.com/photos/texasts/sets/72157618616403212/