in order of priority:
a hearty breakfast
water, in a container you can carry and use in-game.
spare eyepro
good boots
gloves, a zinger to the fingernail hurts like hell
weather appropriate clothing, heavier can help mitigate the sting of bb's, remember that it will likely get mucky or have to wade through spiky undergrowth.
you can whip up a gun and ammo on-site usually even if it means renting if you haven't any battle buddies who can lend, and a spare charged battery is definitely useful even if you'll end up lending it out as often as you use it yourself.
spare clothes are wise but only to avoid the joyous occasion that driving home coated in muck yields having to clean the car the next day.