If the script needs to be run periodically (once a day or hour or
minute perhaps) then something like the unix utility cron would be of
interest, if your script sleeps for any amount of time then cron is
probably the way to go. It's installed on unix boxes by default.

If the script needs to be run continuously then a tool like monit
(which you would have to install) would be worth looking at. It will
keep the script running even if it crashes, it will just restart it if
things go wrong and make sure that it starts when your sever reboots.

Or you could just write a script for /etc/init.d that launches your
script at boot time and pray that it never crashes (which is why I
would recommend monit).