Rule #1 -- Sunscreen
This might seem like a no brainer, but I can not tell you how many times over the last decade that I've called Austin home that I have kicked myself for leaving the sunblock at home. I mean if you plan on doing anything for SXSW, you will have to deal with that big fiery ball in the sky. So slather it on. Many times.
Rule #2 -- Water
Again, this seems obvious. Let me just remind you of what you'll be dealing with. Free parties that start around noon, walking all over downtown and the east side cause parking is a mess, lots and lots of beer, and our old friend from earlier.. the sun. The last thing you want is to have a great day at the beginning of the week, get all dehydrated, feel like hell the next day, and ruin your experience. For the love of Pete, just bring a big old jug of water with you and keep filling it.
Rule #3 -- Scarf/Bandana/Hair Elastic
Ok kids. It will be hot. You will get sweaty. Your hair will feel like straw at the end of the day if you don't take care of it. I'm all for looking cute while running around in the chaos, but you can totally still be cute while being nice to your hair. I would suggest washing and drying your hair and using as little product as possible. A nice leave in conditioner is great, and some even have an spf. Put it up in a ballerina bun or tie a pretty scarf in it to keep your hair from getting windblown and frazzled.
Remember these very basic rules, and maybe this year your hair and skin won't be mad at you after SXSW. Healthy hair and skin is happy hair and skin, and happy hair and skin is pretty hair and skin.