I know a lot of folks will be doing the Warrior Dash for the first time this year, so I thought I'd offer some tips from my experience of running it last year.
1) Either wear tennis shoes that you don't mind donating, or wear an older pair that you don't mind getting seriously messed up. Either way, bring an extra pair of shoes to change into. Last year, there weren't facilities available to change clothes afterwards, but you might bring an extra shirt and shorts anyways, in case you can find a place to change.
2) Bring towels and maybe even wet wipes to help you clean off after the race. The "shower" that was available last year was like a fire hose sprayed at you from a distance, and you only got a couple of seconds in front of it. No doubt you'll still be dirty afterwards, so bring a towel.
3) Lay down towels or garbage bags in your car for the ride home. You're still going to be dirty.
4) Bring your ID and some money for food and activities. I'd recommend bringing a backpack or other small bag to keep all your belongings in during the race. You'll be able to check it in and pick it up after you finish your wave.
5) Wear sunscreen. You're going to be spending a lot of time outside.
6) Plan to arrive AT LEAST an hour early. You never know how bad parking will be, or how long the shuttle will take. Carpool if you can since parking costs $10 a car, and it might be hard to meet up with your friends otherwise.
7) Make sure your bib is attached well and in the correct place. They'll be taking tons of pictures along the course and it will be fun to look at them afterwards, but you have to make sure you can be identified in them.
Let me know if you have any questions about topics not covered above and I'll do my best to answer them!