Comfortable shoes: One of the uncomfortable realities of a con is that you'll be walking or standing all day long. Sure, you might pop a squat in a hallway while you wait in line for a panel or autograph, but overall you should be prepared with comfortable (read: broken in, with decent arch support) kicks.
Light layers: In cities like San Diego, the mornings and evenings are cold but the days get hot. The convention centers tend to be highly air conditioned due to huge crowds, so bring a light jacket in case you get chilly.
A water bottle: Holy crap, I cannot emphasize this enough! There are refreshment kiosks everywhere, but unless you want to spend $4 every time you get thirsty, bring a bottle to fill at the convention center's fountains. Trust me — you'll get thirsty when you're running a mile a minute for the entire weekend.
Snacks and caffeine: On the same note as the water, you'll get hungry (and tired) frequently. Con days are like football minutes: they last forever and so much happens every second! Bring some granola bars and a canned espresso drink to keep your energy up.