I posted some of this in my Fun^3 Fest 2008 wrap up.  I cannot stress the importance of having the right gear during SXSW.

Useful Festival Gear (for the whole Fest):
So I noticed that very few people are prepared for an all day outdoor festival. Granted, the door people search your bags for weapons, booze, and food. But still, that doesn’t mean that you shouldn’t pack a few things. This is what was in my bag (FYI: This year I am testing out the KEEN skyline.  It’s a kid’s bag but I looks like the right size for the job):

• Tissues – also useful when the port-o-potty is out of TP
• Hand Sanitizer or Moist Towlettes, wet wipes and/or face wipes.
• Visine/Eyedrops
• Extra Sunglasses
• Earplugs – I prefer the reusable swimmers earplugs. I bring disposable ones too, it is a good way to make friends.
• Camera – with extra memory (note that cameras are not always allowed at festivals but you can check ahead of time).
• A descent size bag to carry all your stuff and free swag (your friend may or not be willing to be your sherpa). I carry a messenger bag. M usually has her cycling backpack.
• Cash – not all events take credit or have an onsite ATM (It’s also nice to tip the bartender for those free drinks).
• Empty water bottle – staff might prevent you from taking in water, but they can’t stop you from bringing an empty bottle. You can usually find a water fountain or restroom to fill it up.
• Coozie- Most of the time the beer chicks are somewhere in proximity distributing them. Never when you need them though.
• Chapstick
• Sweater/Sweatshirt- It’s good to dress in layers. By the end of the night you will be glad you packed it. I usually stick a hat in my bag for that reason.
• Bicycle- It is the best way to get around town. Driving is not really an option.  Public transit isn’t reliable.
• Walking Shoes- Go practical before cute.  You will be standing long hours on concrete most likely.

Useful Festival Gear (for SXSWi):

• Laptop- Ever so important for blogging on the go.  You can attempt it on your smart phone, but really, a keyboard is your friend.