Good practice in Magento theming

There are a number of points that a good Magento theme will typically abide by:

•  The use of only one layout file called local.xml, where any layout updates specific to the custom Magento theme are declared. Additionally, it is considered bad practice to create any layout files with the same name as any layout file in Magento's base theme.

•  Similarly, there should be no CSS files within your theme's skin that use the same name as any CSS file in the default skin.

• Restricting any .phtml (Magento template files) to just those which have been changed for use with the custom theme. Adhering to these guidelines of Magento theming make it easier to upgrade your Magento installation without making changes to your theme files.