If you haven’t heard, Apple has had some significant problems with their MagSafe laptop power cables becoming damaged or wearing out prematurely. So much so that they have halted shipments on the current model. They are expected to come out with a revised cable soon.
In the meantime, Apple has posted two knowledge base articles on the subject of properly conecting/disconecting the cable, and reducing strain. I recommend checking them out if you own one of these cables.
Apple Portables: How to disconnect the MagSafe power adapter
Apple Portables: Reducing cable strain on your MagSafe power adapter