Don't get picked off one by one.
Having someone watch your back comes in handy when saving seats, waiting in line, and keeping you and your belongings safe.
Wear Comfortable Shoes:
You never know what lurks around the corner.
A bowel movement can sneak up on you just as fast as a flesh eating zombie or axe wielding murderer. Also when you're standing in line, walking the convention floor, or posing for cosplay pics, you're gonna want shoes that won't murder your feet.
Expect The Unexpected:
Screaming doesn't scare off them off. It just gives away your position.
Don't have a freak out when you see an actor or celebrity walking around the con. Always be polite. Also be understanding if they don't have time to sit and chat for an hour as they're probably en route to an interview or panel.
Don't wake the beast.
Don't push the wrong person's buttons. Those cosplay swords won't cut you, but they'll still hurt. Talk at a volume that is respectful to those around you, and in some cases just keep your trap shut. There's nothing worse than not being able to hear a panel you've waited hours to see because the people around you can't stop chatting. This also goes for waiting in line overnight when a majority of people are trying to sleep.
Bring Your Own Snacks
It's a trap!
As tempting as the hot dogs, cookies, pizza, and chips are, the vendor food is murder on your wallet and on your stomach.
Don't Record What They Tell You Not To:
Someone knows you watched the tape... and what they say is, "You will die in seven days."
Don't tempt the evil spirits… a.k.a. the private security with night vision and infrared cameras who will bust your ass if you try to film something you shouldn't. Studios bring a lot of exclusive footage to Comic-Con. That's what makes the experience of actually being there so special. You can brag about what you saw, just don't film it and put it on the internet.