non-cms page

To your local.xml file (which you should be using rather than editing individual xml files), add:

<layout>
	<contacts_index_index><!-- the page to be edited, found in its original xml file -->
		<reference name="content"><!-- Structural Block where placing Static Block -->
			<block type="cms/block" name="your_block_identifier">
				<action method="setBlockId"><block_id>your_block_identifier</block_id></action>
			</block>
		</reference>
	</contacts_index_index>
</layout>

If you already have an existing local.xml, you won’t need to include <layout>. If the page you want to edit already has existing changes in local.xml, you only to add the <reference> bit and its contents.