Socks on water bottles
Got a water bottle you're trying to keep cold? This one holds up just as well on the homefront as it does on deployment. Soak a sock in some cold water before you head out, and then toss a water bottle in it. It'll help keep it cooler for longer. Sure, it might make the outside of the bottle smell like a McDonald's Playplace, but if it keeps you hydrated—it's worth it. Which brings us to point number two...
Seriously, drink a ton of water
The old adage "if you're thirsty then you're already dehydrated" is a wise one to live by this summer. Soldiers hauling 60 pounds of gear in 90 degree weather (while blanketed in insulated cammies) can't stay cool—their only option is to drink an assload of water continually throughout the day. It's usually recommended to drink 1 1/2 quarts of water per hour to avoid "heat injuries" such as heat stroke. Your pee shouldn't be the color of a Lakers jersey. It should be the color of, uhh, nothing.
Set up a shady canopy
Look, if you had... one shot, or one opportunity, to make your patio a little cooler outside, would you canopy it, or let it spit-fry? Your palms are sweaty. Sure, that's understandable. Your knees are weak (from heat), and your arms are heavy (also from heat). If there's vomit on your sweater already, you are suffering from heatstroke and should contact medical services immediately. Don't be nervous, just be calm and ready. Sometimes a little bit of shade, also known as slim shady, goes a long way.
Dunk your arms in an ice bucket of water
Everyone knows that you can hit an ice bath to drastically regulate your body temperature. However, if you're too hot, the extreme change in body temperature can actually send you into shock. To mitigate this risk, some on-base soldiers will roll up their sleeves and dunk their arms into an ice bucket (sometimes called an "Arm Immersion Kit" by higher-ups with too much time on their hands) full of water and allow them to soak until their blood temperature drops a bit.
Jury rig a ghetto A/C unit
What you see before you is the latest innovation in hood engineering. Many a budget-restricted renter has pulled off a MacGuyver A/C attempt, but none succeeded like this anonymous Twitter user. Put this baby on full blast, grab a cheap beer from the back of your (roommate's) fridge, sit in your inflatable mini kids pool (that you definitely didn't steal from your nephew's birthday), and enjoy a freezing blast that rivals the arctic winds.
Slap on some camouflage cream
If you're enlisted, this sh*t is basically free sunblock. This one won't help keep you cool, necessarily, but it will protect your skin from harmful UV rays and prevent sunburn. Not to mention it can make you look like an intimidating linebacker, an overrated 60s rock guitarist, or Arnold Schwarzenegger—depending on how you apply it.