Some silly advice:
If you see two people in Lina and Gourry costumes, never follow them. NEVER.
If the Sailor Senshi who are approaching you all have beards and beer guts, it is considered impolite to run away screaming.
Some serious advice:
Always respect everyone's personal space! In a crowded event, personal space is at a premium. Never touch ANY part of ANYONE'S person or costume unless you have their permission.
If you are going to be meeting a girlfriend or boyfirend at the con, or hoping to find one, remember the difference between public and private behavior. Nobody else really wants to watch you make out.
If you DO meet another consenting adult at the con, make sure you have protection, and use it. I remember a sci-fi convention where they handed out condoms to everybody who showed up and looked like they even might be of age. Nobody wants to come home from a convention with a disease -- or an unplanned pregancy!
NO MEANS NO! NO ALWAYS MEANS NO! If someone insists on pressing their attentions on you after you have refused them, that's felony assault. GET AWAY FROM THAT PERSON IMMEDIATELY, no matter how you have to do it, and CONTACT SECURITY IMMEIDTALEY! Try and remember the person's bagde name so the organziers can deal with them appropriately. (If I were the concom, I'd be kicking them out and handing them over to the cops if they tried that stunt....)
Always treat minors with respect. Don't assume they're unknowledgable because they have an under 18 badge. just because someone is dressed as Misty doesn't mean they don't like other anime too.
It really IS OK to like Poemon or Yu-Gi-Oh!, so don;t insult people for expressing a liking for those shows.
If a behavior would be unethical anywhere else, it is also unethical at a con.
If you require medication for any condition, be it medial or psychiatric or whatever, take your meds with you to the con and maintain your usual dosage schedule. Your health is more improtant than anything else while you are at a con. You can't enjoy the con if you ignore your health! Also, have a plan in case you have a medical emergency and make sure your friends know it in case you lose consicousness.
If you use alcohol at the con, use it responsibly and only in a location where alcohol is permitted. And never use illegal or street drugs at a con -- this could get the roganizers, as well as you, in a hgue amount of trouble, and we really don't want that.
Make sure you get adequate food and fluids. Again, it comes down to doing what is neccesary to keep your health and strength up so you can enjoy the con.