TAKE A SHOWER! If you're camping on-site, you can take a shower right there on the festival grounds (on-site campers have access to the "shower trailer"). If your crew rents an RV, many of them have on-board showers. But for most of us, this means taking a shower back at the hotel room, so wait to take one right before you get in the car to drive to the festival. Use lots of deodorant!
DRESS APPROPRIATELY. Some people go for style rather than comfort at Coachella. Beck said it best, between songs during his surprise appearance at the blazing hot 2004 Coachella Festival... "I feel bad for all the Goth kids, dressed in all black, melting in the sun today. I was thinking about going goth this past week, but I decided to put it off until after the festival - for health reasons."
To be sure, if you wear too much clothing in 100+ degree weather, you're gonna be dripping sweat all day, and smelling really rank by nighttime.
BABY WIPES ARE YOUR FRIEND. At your local Target, they have a section for travel-size toiletries where you can find small packets of handi-wipes or baby wipes. Buy a couple packets and toss them in your backpack - they'll come in handy throughout the weekend. Use them to wipe that inevitable layer of dust and grime off your face, hands, arms... hell, even your armpits and "down below" if necessary. They'll even work as a substitute for toilet paper if you find yourself in a porto-potty with no TP. Even better are the larger "bathing wipes" produced by companies like No-Rinse® which you can find at stores like REI and Sport Chalet.
BRING DEODORANT. Sometime around twilight, wipe yourself down with a couple baby wipes and re-apply deodorant. Your friends will thank you when you're pressed up against them for two hours during the headliner. You can find small, travel-size deodorant in that same travel toiletries section at Target.
RINSE AND REPEAT. If you don't mind getting your clothes wet (and if it's 100 degrees again this year, you probably won't) you could always take a couple bottled waters and just dump them over your head. Or, you could head over to the tropical "Misting Oasis" and stick your head in the fountain.