Bring a great hat - it's not just a fashion item, it can make the difference.
Careful where you drive and park - don't get stuck. The Joshua Treenial venues are all on or near paved roads. Take care when you leave the pavement and only park where you know you can get out. Four wheel drive rules.
Bring a flashlight for guidance under our dark skies.
There are critters in the desert - snakes, scorpions and the like. Take care when in rocky areas and keep a good lookout when walking around. At the same time, don't be scared :)
Your participation in the Joshua Treenial is at your own risk. Now go out and have some fun.