In keeping with other RSS feeds, be sure that your output is sometimes badly formed.
Make sure that you don't correctly escape strings.
Pay scant attention to encodings, ideally send the file as one encoding but claim to be another.
This way you will continue to test the abilities of RSS feeders to read rubbish XML.
Once finished, serve with the wrong content type and don't bother with having correct last-modified and expires headers.
Or alternatively, break with convention and actually make correct XML in all cases. Perhaps even take care to use a library which generates correct XML.
Mark