Since most of the files in an Espresso plug-in are XML, you must remember to properly escape the illegal characters &, <, and > either using character entities, or by wrapping the entire element's contents in a CDATA block. For example:
<!-- This will prevent this file from loading! --> <myelement>Remember that < & > are illegal characters</myelement> <!-- Either of these two options will work properly --> <myelement>Escaping with character entities like &</myelement> <myelement><![CDATA[Wrapping in a CDATA block: < & >]]></myelement>
If your plug-in is mysteriously failing to work, remember to check for illegal characters! It also might be worth running your XML files through an XML validator if they are not loading, because XML formatting is a lot more strict than HTML, and thus easier to mess up if you forget a closing tag or similar.
Also, remember that any changes you make to your plug-in's XML will require you to restart Espresso to see the changes.