This is my checklist of stuff to bring:
- Business cards, at least 250 of them (you had one out to pretty much everyone you meet, which is a lot of people in the space of a few days; better to have too many than to run out). In one of the classes I teach, I require students to purchase their own business cards on the pretense of using them for a game design exercise, but the real reason is so they'll have them on hand when they go to GDC.
- Clothes. Casual is fine, most developers will be. If you're allergic to denim or something, business casual is acceptable. Avoid either extreme (suit and tie makes you stand out, and not always in a good way; ditto ripped jeans and a t-shirt with M-rated content).
- Resumes, if you're looking for work. Do not hand these out to anyone, or offer them, or even mention that you have them; it's considered rude. But if (and only if) someone asks you, it's good to be prepared. Likewise, if you're entering a profession that expects a portfolio, making a few CDs to hand out with your resume can't hurt (but if you're on a budget, at least put it on your own thumbdrive and take it with you). If you're a game audio person, it's easier: put your portfolio on your iPod.
- Physical notebook (you know, with actual paper made from trees) and plenty of things to write with. It's easier to take notes during sessions if you don't have to jockey for outlet positioning, and if someone asks you for some information that they can take with them you can write it down on paper and give it to them. You can't do this with a laptop.
- Cell phone. If you meet someone that you want to meet up with again later in the week, the only elegant way to do this is to swap cell numbers. If you don't own one, consider renting one for the week, or buying one of those pre-paid ones (and write your phone number down somewhere easy to access, like on the phone itself, so you'll know it when asked). Oh, and make sure you turn off your phone before entering any session. In two years I don't think I've ever made it through a session without someone's phone embarassing them; don't let that person be you.
- Laptop is optional (unless you're carrying your portfolio on it). Take an extra battery if you have one, just in case you can't sit near an outlet, and turn off the sound.
- Sleep. Okay, it's not something you pack in your luggage, but be sure to get plenty before you go. You can count on having next to no sleep as long as you're there.