It helps me something good to format my code files (.mxml and .as) with a standard procedure, nice plug-in. It supports both standalone Flex Builder and Flex Builder Eclipse Plug-in. Here is the plug-in update site URL for this Flex Formatter.
Please follow the below steps to set-up this plug-in into your Flex Builder. Click the image below and view the steps as slide show.
Nice work. Thanks to Ernest.