1. A large messenger bag. I highly recommend a Bag of Holding from ThinkGeek (be sure to pimp it out with something uniquely you so it stands out against all the others that will be there!) but any large messenger bag will do. I do not recommend backpacks as they are not easy to get into when you're trying madly to stuff your swag away in a crowd or trade buttons on the fly. Plus, they are much bulkier, which won't be appreciated in a packed line or crowded room. Messenger bags are easy to get items into and out of, easy to store under seats or in your lap, and easy to flip into different positions around your body when you need to be a ninja.
2. Nintendo DS. I am not listing the PSP as a portable gaming option here because, unless you are going to a 75 thousand plus convention just to be anti-social, you're not going to need one. A DS is an instant way to make new friends and entertain yourself in the gigantic lines, will be your lighter at the concerts, and is needed to witness the stream of inappropriate penis images flying through the air in Pictochat.
3. Snacks. Trust me on this one: There will not be a whole lot of time to eat. You'll want to show up for lines at least a half hour early, panels are each an hour long or more, and a lot of the time you're going to be running from one place just to barely catch the line for your next panel, make a tournament, or meet up with new friends. I almost starved to death my first year! Now, in my wisdom, I will be packing a handful of Tiger's Milk bars, which are full of vitamins and calories. Having something small with a decent amount of calories to munch on so you don't have to waste time waiting in yet another line of PAXers at a restaurant will keep you from collapsing from exhaustion. A bottle of water helps too!
4. Chargers and/or extra batteries. At all times you should have with you every charger or extra battery for every electronic device you have on you. Expo days are long ones, so you might not have a lot of time to sit near a wall or go back to your hotel room to plug in. This year I am actually getting a Universal Gadget Wrist Charger so I can be cool and hip while charging my phone!
5. Hand sanitizer. When you have lots of people all touching the same controllers and systems for game demos, the germs are going to build! There's no need to douse yourself in sanitizer every time you touch anything, but it's a good idea to exterminate the germs on your hands before you eat, touch your cell phone or computer, and after shaking hands (though shaking hands is now forbidden). Washing your hands is much better, since you kill the good germs along with the bad ones with alcohol based sanitizers, but when you can't get to a sink a little bottle of the stuff won't hurt!
6. Umbrella or layered clothing. Outside of the convention, you are in a gray, cold, rainy city. Inside the convention, you are in a sauna! Be sure to pack a small umbrella with you, or at least a sweater. If you wear a light jacket over a sweater over a t-shirt, you can easily strip up and down as you come in and out of the convention center. However, as any Seattleite will tell you, a little rain won't hurt. ;)
Over time you'll put together your own personalized PAX survival kit, but I hope these suggestions help you get it started. With a little planning, you can survive PAX like a pro and emerge no worse for the wear!
What about you other regular attendees? Any suggestions for the newbies out there?