fishlips,
I've rattled in a few muleys through the years. The key is timing. It's got to coincide with the rut. One thing i've found that helps is you want to kinda rattle the tips of the horns,you dont want to smack em together like they do for whitetail. You want to do some ground scraping with the horns,make it sound like deer are pushing each other around.Then mix in some grunts and every now and the some bleats and remember where there's deer, theres deer scent, so don't forget to put out scent. If you have a area already picked out then once you start gettting close to that area use scent drags,any deer that hears the commotion and happens to hit that scent trail knows there's some hot doe somewhere in that area.
Good Luck!