<config>
    <placeholders>
        <my_placeholder_identifier>
            <block>mycompany_mymodule/blockname</block>
            <placeholder>MY_PLACEHOLDER_NAME</placeholder>
            <container>Mycompany_Mymodule_Model_Container_Blockname</container>
             <cache_life>0</cache_life>
             <cache_lifetime>0</cache_lifetime>
        </my_placeholder_identifier>
    </placeholders>
</config>