<config>
    <global>
        <events>
            <event_name>
                <observers>
                    <unique_name>
                        <class>Class_Name</class>
                        <method>methodName</method>
                    </unique_name>
                </observers>
            </event_name>
        </events>
    </global>
</config>
<global>
        <events>
            <log_log_clean_after> <!-- event name -->
                <observers>
                    <catalog_product_compare_item>  <!-- unique event code -->
                        <class>catalog/observer</class>
                        <method>catalogProductCompareClean</method>
                    </catalog_product_compare_item>
                </observers>
            </log_log_clean_after>
        </events>
</global>
- See more at: http://excellencemagentoblog.com/question/register-an-observer/#sthash.HxccSA1m.dpuf