Here's what I recommend:

1) Hat with a brim all the way around, to keep the sun out of your eyes and off your face/neck
2) Dark Sunglasses
3) Sunscreen
4) Insect Repellent (depends on where you fly)
5) Cell phone turned on and on your person (pocket or belt)
6) First aid kit

At first I just wore baseball caps and sunglasses. After I realized I was getting too much sun, between my flying time and other outdoor activities, I decided to start using sunscreen and a full brim hat. I personally don't like the look of a full brim hat as well as baseball caps, but I'd rather not have sunburned neck/ears! I agree with onewasp, do what you can to protect against those UV rays! First aid kit can take care of small mishaps/injuries, while the cell phone will come in handy if there is a serious injury. Insect repellent is probably the most recent addition to my list, since a small local field that I fly at often has some pesky bugs on occasion (mosquitos, gnats, etc).