-Wear comfortable shoes
-Drink lots of water
-If you wear a costume, make sure you can be comfortable in it all day, and if it has a weapon or other accessory, it will get really annoying about 2 hours in
-If you see someone in a cool costume and want a picture, just ask, they'll gladly pose for you - don't be a creepy guy that tries to surreptitiously takes pictures, you're not fooling anyone
-If you plan to go to panels figure out where they are ahead of time and get there early, popular panels can fill up and you may be stuck in the back or in the hall and not get in at all
-Have a budget of what you're willing to spend at vendors, if you don't, you can easily spend way more than you intended. A good way to avoid this is carry only the cash you're willing to spend, leave your credit card at home (plus vendors will charge sales tax if you pay by card, but won't worry about it if you pay with cash)
-Finally, and most important: Have fun!