If you are planning on NPCing then bring comfortable black clothing.
If you are planning on PCing then make sure your costume is comfortable and warm/cool based on the weather.
Footwear is the often loverlooked important part of the costum and must be comfortable. If your feet hurt because you are running around more than you noramlly do, you will not have as good an experience.
Likewise, packing an extra pair of socks, no matter the weather is always good advice.
Bring sleeping gear for overnight stays. Some locations we use have sleeping cabins that have bunks with thin matresses and no sheets or pillows.
Sleeping bags and pillows are a must for the overnight (most) events.
Overnight bag (toothbrush and such)
Snacks - while some snacks and meals are provided, you are burning a ton of calories, and might need something to take off the hunger edge.
I'm sure others will also help out with opinions and suggestions as well.
Hope to see you starting next season,