Er, people already pointed out lots of things so I'll just add some stuff / repeat some stuff...
Watch your step! Meaning don't step on someone's costume. Or whack someone with your prop, especially if you have a big one.
Don't block places where people pass a lot... meaning halls, stairs and otherwise.
If you have any doubts about whether or not your prop is permitted, contact the con (or the bus company or the plane company, depending on what your doubts are about- they will be able to tell you about the rules).
DON'T FORGET TO EAT.
DON'T FORGET TO BATHE (at least some deodorant.)
DON'T FORGET TO DRINK (water).
And the thing I regret not doing the most for cons... CHARGE YOUR CAMERA. And your phone.