Frequent job scheduling like cron

[Michael Hipp]

I have a job I need to run automatically at about every 5 minutes. Cron 
could certainly do that. But if the job should run long, I don't want it 
to be started again while a previous instance is still running. And if 
the job should run 4.5 minutes, I don't necessarily want it to run again 
in 30 seconds, tho that wouldn't be fatal.

I could just use a script with a delay at the end before it loops back 
to the top, but that's crude and not particularly reliable.

Any clever, robust way to do this?

Thanks,
Michael