Truthfully, if your mask is fogging, you need a new lens or a new mask. Thats the first and most important item to have in your referee wardrobe. You wear it all day often for well over an hour or so at a time. it has to be comfortable, should usually have a tinted lens (so players can't see your eyes, though this is geared more towards tournaments, and also so that the sun is not hurting your eyes all day). Fans work sometimes, but typically aren't really worth it. Makes the mask heavier, batteries die, etc.
Drink lots of water starting a day or so early. Take in plenty of salt. Carbs and greens the night before when possible. Some fluids will help hydrate you better. I love coconut water but it isn't for everyone. I drink it with pineapple juice added and the little bit of sugar is a great pickmeup on a really hot day.
Some small proteins at the field such as nuts or jerky are a nice added touch. Cotton T-shirts work if you aren't wearing them under something, otherwise, your under layer should be something moisture wicking. Minimize layers. Tank top, jersey, armpads, single layer on head, shorts if possible. Pants are heavy and prevent a nice breeze across your leg.
A frogg togg cooling towel over your neck is a nice touch if you have easy access to a cooler.
For every gatorade or sports drink you consume, drink 2 bottles of water.
Pickles are delicious and the vinegar isn't a bad thing on hot days as well.
Mustard has decent amounts of potassium. If you find yourself cramping, try eating a mustard packet or two. Its gross but it can help.
CFOA Referee 2012 -
PSP Referee 2013 - 2015
MiLP Referee 2014 -
NXL Referee 2015 -2015