Have only been going for 3 years running, so hardly a vet, but I've learned a few things...
-Bring your own food, and do not attempt to eat it anywhere on the exhibition floor. I stopped to do such, and got yelled at by like 5 security people.
-Respect the security people. They are mostly nice, but you get a few angry ^^^^^es who will not compromise with you. Respect them, and just walk away, or you will get shanked.
-Please watch where you are walking. I know there's a lot to look at, and it's pretty much shoulder-to-shoulder, but there's no excuse for elbowing me in the tits because you can't pay the slightest attention to where you're headed.
-It's been suggested already, but FRICKIN SHOWER. I've met some of the nicest people at SDCC, but it's hard to want to carry on intelligent conversation when you're talking to someone who smells like they haven't bathed in weeks and are marinating in their own sweat.
-Meet people! Introduce yourself to people with similar interests! Last year, when Husker75 and I were waiting around for a panel to start, a noobie and her daughter asked us about how early they should arrive for the 24 panel. We ran into them again at the panel, exchanged contact info, and we were invited to their house for an awesome 24 tv-movie watching party with other 24 geeks.
-If you want to take a picture with that squadron of stormtroopers or that chick dressed like a Replicant, go for it and ask! I was way too shy my first year and missed some hella cool photo ops.