1) If you invite a guest to your Con, do not fail to make sure his or her experience is pleasant. Keep in mind that fandom includes (embraces, even) many individuals with less then stellar social skills. Fandom wouldn't be fandom without them. I would even hazzard to say ALL of fandom comes under this title to one degree or another. In particular, some people don't know when they are not welcome anymore. Make sure you give the guest a means of escape he/she may be too polite and/or inexperienced to make for themselves. Newly popular guests would be the most vulnerable. NEVER forget that. You don't have to drag them off (usually...although that can be fun.) but offer an escape and leave it up to them to accept or reject it.
2) If you are in a group around a popular guest who is telling really interesting/funny stories, DO NOT INTERUPT HIM/HER. You will only manage to piss everyone else there off. IF (it's a big if) the story you just HAVE to tell is short, entertaining and doesn't cause the guest to stop talking, you may just be forgiven. On the other hand, if it causes the guest to stop talking, to move on, is too long, or at all boring (or, worse yet, all of the above), expect death threats.