This may be old news, but I thought it was a worthy suggestion to prolong the life of any Mac power adapter cord that has the small u-shaped sliding cord keeper. The idea is useful enough that it might save Apple some complaints about their shorted-lived cords.

Slide the cord keeper right up to the reinforcement at the male end, as seen in the image at right. Insert the plug into your Mac. About three inches from Mac, make a loop and double the cord back on itself and insert the doubled portion into the cord keeper to maintain the loop. This reinforces the connection to the computer, holding it at a 90 degree angle from computer. It also serves as a shock absorber, as the loop will release first before pulling the cord from the computer.