I am a true believer in the Mean Girls Halloween philosophy: "Halloween is the one night a year when girls can dress like a total slut, and no other girls can say anything about it." I don't normally condone dressing like a whore, but on Halloween, anything goes. Halloween is the time to be something you're not, so if you want to dress up as a sexy nun or a sexy nurse, go ahead. No one has the right to judge you on that night.
While I'm not going to judge you for dressing on the skankier end of the spectrum, I've got to caution you. Girls, wear clothing that fits you, please. There is nothing worse than seeing a girl with a serious belly in a midriff shirt or a girl with enormous thighs in a miniskirt. I know it sounds harsh, but guys are harsh, and they're the ones you care about judging you. Just because a costume is sexy doesn't mean you look sexy in it. Try on the costume before going to the party and be honest: Do you look good in it? Does it show off your best features? If the answer to either question is no, go for another costume - for your sake and for the sake of everyone who has to look at you.
Guys, this goes for you too. While being humorous is good to a point, no girl wants to look at your beer gut, no matter how proud of it you are. Judge for yourself, but if you're comfortable looking like a total idiot and not getting any that night, more power to you.
In terms of shoes, guys get off easy - unless they're dressing in drag, they're wearing flat shoes. Girls, my best advice is be comfortable or be warned. If your heels are painful but make you look hot, you can make the sacrifice, but you'll have no one to blame at the end of the night for your aching feet but yourself. If it's any consolation, I'm sure I'll be in pain with you. After all, beauty is pain.
Halloween is great because we get to be who we're not, but it doesn't stop people from judging the way you look. Dress to play up your best features, be comfortable and have fun; if you spend your night worrying about sucking in your stomach, your holiday will be ruined. Happy Halloween!