How to set the default sort order for catalog pages?

If you only want to change default sort order you can do it in catalog.xml just by adding the following action:

<action method="setDefaultDirection"><dir>desc</dir></action>

This action will work in any block of type “catalog/product_list_toolbar”, so the XML snippet would be:

<block type="catalog/product_list_toolbar" name="product_list_toolbar" template="catalog/product/list/toolbar.phtml">
        <action method="setDefaultDirection"><dir>desc</dir></action>
    <!-- The following code shows how to set your own pager increments -->
    <!--
        <action method="setDefaultListPerPage"><limit>4</limit></action>
        <action method="setDefaultGridPerPage"><limit>9</limit></action>
        <action method="addPagerLimit"><mode>list</mode><limit>2</limit></action>
        <action method="addPagerLimit"><mode>list</mode><limit>4</limit></action>
        <action method="addPagerLimit"><mode>list</mode><limit>6</limit></action>
        <action method="addPagerLimit"><mode>list</mode><limit>8</limit></action>
        <action method="addPagerLimit" translate="label"><mode>list</mode><limit>all</limit><label>All</label></action>
    -->
</block>

This block is used twice in catalog.xml once for normal product lists and once for layered navigation product lists, so just make the same change in both places.

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<action method="setDefaultOrder"><field>entity_id</field></action>