How to determine if a script instance is already running?

I have a command line script that needs to run continuously, and so I plan to have cron execute it every so often. I want to have the script first check to see if another instance is already running and, if so, die(). Now, I know I can exec the process list and parse through the output, but is there and easier, faster, cooler, sexier, better way? :o) JM