You encounter this most often with folk that don't have a backup copy. There is no known maintenance that will prevent such failures but you can prevent loss with the old fashioned backup copy.
I'm sure someone has written long articles about this.
If data recovery is required, CLICKERS, if putting the drive into a new enclosure does not help means we send the drives off to places like http://www.drivesavers.com
I am currently working with some photographers with years worth of back up images on external hard drives. They have heard that occasionally booting up these drives (some of which have not been used in years) and clicking on a fw random files can help the drives last longer. Does anyone know if this is true or if their are any other similar measures we can take to maintain these drives? thanks!