You can use the watch command, but it unfortunately isn’t available for Mac OS X. At least, from Apple. Sveinbjorn Thordarson (great name!) has a version of watch that you can download and compile on your OS X box. It’s available at http://www.sveinbjorn.org/watch_macosx.
Or, you can use this shell script:
while true ; do foo ; sleep 1 ; done
This will run foo every second until you press Ctrl-C to cancel the script.