1) Wear nothing but white clothing, in the lightest weight of fabric you can find.
2) Find the shade and hide in it as much as possible.
3) Reconsult your schedule, say "screw it" about all of the lesser known bands, and show up at around 6 p.m. for the main stage folks instead.
Sadly, we are made of fail, ignored all of those tips, and left from White River before 7 p.m. Also, because of stupidity of epic proportions (not on our part), we missed Black Tide. :( So we saw Airborne and Walls of Jericho, and the very beginning of Mastodon's set. Then we went and spent too much money for food, and decided that sticking around for Disturbed was just a poor choice all around. So we went to the mall instead, because it's air conditioned. :)
Not exactly the best story of things, but MAN, it's hot in the direct sunlight. We Seattle-ites don't know what to do when the sun shines straight down all day long! :)