1. Comfortable shoes. Forget the stilettos. The conference is now broken out into themed campuses. Your sessions can be up to a mile apart. Parties always seem to be located at the top of a hill.
2. Want to be friended by people around you? Bring a power strip. Every session fills fastest near wall outlets and people are incredibly grateful for shareable connections.
3. Extra batteries for whatever needs batteries.
4. Water bottle. Alcohol is everywhere, water not so much.
5. Dollar bills. For parking. Instead of housing your rental in a mindblowingly expensive conference hotel lot (which are filled to capacity by 10 AM), you can leave it in open lots all day for $7 if you've got enough dollars to poke through the slots of these low-tech machines.
6. A thumb drive. Or two.
7. Business cards with white space to write on. So people can make note of where they met you. Trust me. They'll never remember.
8. Patience, good will and a sense of humor. There's expected to be 20,000 campers besides you. Real world social skills make the experience better for everyone.