Apple Portables: Reducing cable strain on your MagSafe power adapter
Summary
Proper usage and maintenance of the MagSafe power adapter can substantially increase the life of the adapter.
While the MagSafe power adapter is designed to travel with your portable Apple computer, certain usage patterns can contribute to the fraying or breaking of cables. The MagSafe adapter, like any other metal wire or cable, is subject to becoming weak or brittle if repeatedly bent in the same spot. Follow the recommendations below to reduce the likelihood of damaging the adapter.
Inspection
The most important way to maintain adapter health is to regularly inspect the cables, connectors, and the adapter itself for any kinks, breaks, bends, or other damage. Should you find any such damage, discontinue use of the adapter.
Note: Even if your product is out of warranty, you can take your adapter (you don't need to take the computer) to an Apple-Authorized Service Provider or Apple Retail Store for evaluation. Replacement of the adapter will depend on warranty status and evaluation.
Products Affected
Portable Computers
Setting up and using the adapter
When unwrapping the cord from the adapter, be sure to support the weight of the adapter (Figure 1). If the connector is held and the weight of the adapter is used to unfurl the cable (Figure 2), the cable or the adapter itself could be damaged.

Figure 1 Unwrapping the cable while supporting the adapter

Figure 2 Unwrapping the cable by letting the adapter fall
Excessive bends in the cable may ruin the cable over time. The most likely points where excessive bending may occur are near the adapter brick (Figure 3), or the MagSafe connector itself (Figure 4). Ensuring slack in the cable at these particular points (Figure 5) preserves the durability of the adapter. Aim for gentle curves instead of angles. Proper usage increase its longevity.

Figure 3 Bending by the adapter brick

Figure 4 Strain at MagSafe connector

Figure 5 Slack at MagSafe connector
If you are using the two-prong 'Duckhead' connector for your MagSafe adapter and you find the cable barely reaches, switch to the three-prong, grounded connector to improve the reach of the cable and reduce strain. Figure 6 shows the length of the adapter with the Duckhead and Figure 7 shows the length of the adapter using the three-prong, grounded connector which is much longer.

Figure 6 Two-prong Duckhead connector

Figure 7 Three-prong, grounded connector
Disconnecting and storing the adapter
Properly disconnecting the MagSafe connector from the computer can prolong the life of the adapter. Using the connector itself instead of the cord is discussed further in Apple Portables: How to disconnect the MagSafe power adapter.
Apple portables have the MagSafe connector on the left side of the computer. If the cable for the adapter must be run to the right side of the computer, run the cable around the back (Figure 8) instead of under the computer (Figure 9). Make sure there is plenty of slack on the connector side and that the connection is not strained (Figure 12).

Figure 8 Cable run around the back of the computer

Figure 9 Cable run under the computer
Always keep the area around the adapter and its cabling clear. Some items that come into contact with the cable could cause significant damage. The cable shown in Figure 10 is likely to be run over by the wheel of the chair which could cause a break inside the cable.

Figure 10 Cable likely to be damaged by objects in the workspace
The 'wings' (Figure 11) allow the cable to be stored with the adapter using a very small footprint. Note that improper wrapping of the cord on these 'wings' may cause issues similar to bending the cable. When wrapping up the cabling, be sure there isn't additional strain on the cable where it connects to the adapter brick. Figure 12 shows excessive stress and Figure 13 shows proper wrapping. Wrap the cable loosely to avoid creating undue stress or harsh angles with the adapter.

Figure 11 Wings open to accept the cable

Figure 12 Excessive strain on cable when wrapped

Figure 13 Properly wrapped cable
If the computer is being used regularly in the same locations, consider obtaining an adapter for each location. Having a dedicated power source in each location limits the need to setup and store the adapter on a frequent basis and helps avoid potential strain relief issues all together.
Additional Information
If you have questions or concerns regarding other adapter-related issues, please refer to Portable Computers: Troubleshooting power adapters. Even if your product is out of warranty, you can take your adapter (you don't need to take the computer) to an Apple-Authorized Service Provider or Apple Retail Store for evaluation. Replacement of the adapter will depend on warranty status and evaluation.
For information on properly disconnecting the MagSafe power adapter, see this article.