Thought of some more after Mikomicon:
For those cons that are Open Mic:
Do know your lines before you go on stage. DON'T ad-lib it. It's obvious to the audience (however small) that you're making it up, and unless you're a master at improvisation, it'll come off as weird. Also, your groupmates will be confused and it'll turn into a trainwreck until you figure out how to get back on track.
Do make sure to address special issues before the Masquerade starts. As Assistant Masq. Director, I was in charge of making sure the actors were ready, and I swear, right before they were about to go on, I was alerted about lighting, microphones, lack of stage ninjas, and group problems. Accidents happen, of course, but most cons have a Masquerade Meeting on the day of the masquerade to talk about these things. Address those problems then, and try and think of anything you might need, even if you don't think there's anything. Because I promise, five minutes before the show, you're going to think of something.
Do be orderly as you exit. Before the Masq, plan how you're going to exit, carry off your props, and return anything to con staff (microphones, etc). It's okay to grab props while you're still on stage, then exit - just don't leave them on there. Don't run off with things the staff has given you - I had quite a few skits run off with our mikes accidentally. Which ended up in me running across stage and through the audience to get them back. Funny, yes, but slows the show.
Don't panic, and do remember that mistakes happen. No human is perfect (sadly), and you're going to make mistakes. The most important part is to not panic!! I can't stress this enough. If someone loses a prop, or a part of their costume on stage, don't pause to pick it up, especially if you have a prerecorded track - it'll be completely weird. Stay in character; try and have someone who's not about to speak push it aside. The audience will get the idea. Don't panic so much before the show, either - accept the fact that you'll make a mistake, and be happy that you can experience going out there and performing!
~-Anime Los Angeles 2010~
Cosplays In Progress
Hachikuji Mayoi (Bakemonogatari) : Wig (68%)
Rakka (Haibane Renmei) : Planned for AX2010
Days 1, 2 and 3
Hachikuji Mayoi, Yoshika Miyafuji / Haruhi Fujioka (one or the other)
Staff Member, Panel Participant, Ribbon Giver