Expect to sweat- a LOT- sweat running down your shins, down you back into your underwear, from behind your knees, on top and bottoms of your feet, etc. If you shower mid-day there's no way you are crawling back into your morning clothes, so bring two complete sets of clothes for each day- from the skin on out!
We found it was hard to drink enough water to keep up with our needs. After awhile of drinking just plain water, we found ourselves feeling bloated and icky, but soda isn't an option for us; nobody likes it. We've taken to carrying the little tubes of powder that you put in drinks or the small bottle of squeezable drops, so the water has a different flavor and is more palatable for us.
I will second the idea of wet washcloths. I usually freeze soggy facecloths overnight in zip top bags, then stash them in an insulated sandwich lunch bag and carry them in my backpack. It's nice to have something cold to wipe off with... and you can get a cup of ice from the QS stand to put into the washcloth bag if it warms up too much. I found some personal fans in
Walmart, that you wear around you neck- they're about the size of an iPod. They are pretty effective in blowing cooling air on your neck and face. I think they were about $5 each.
We also use spray antiperspirant on our feet in the morning, to keep them from sweating, chafing, and blistering. It's not 100% effective, but it sure does help... at least for awhile!