You can get 500 bonus US Airways miles when you download their web browser toolbar called the Dividend Miles StoreFront Assistant, install it, and make at least one search. Note that some people have reported getting the miles with just a download, no installation required. In any case, you can uninstall it later.
To qualify for the initial download 500 bonus miles offer, make at least one search or purchase using the Dividend Miles StoreFront Assistant. Bonus offer is valid only for the initial download and install of the Dividend Miles StoreFront Assistant. Not valid on subsequent downloads or installs.