Go to your themse’s local.xml and place the code

<catalog_category_default>
          <reference name="product_list">
            <action method="setDefaultDirection"><dir>desc</dir></action>
          </reference>
        </catalog_category_default>

        <catalog_category_layered>
          <reference name="product_list">
            <action method="setDefaultDirection"><dir>desc</dir></action>
          </reference>
</catalog_category_layered>