• Drink A Lot Of Water - This should go unsaid, but many forget! Rule of thumb for how much you should drink a day, take your weight in numbers, divide by 2 and that's how many ounces you should drink a day, plus x2 however many ounces of coffee or soda you drink.
• Wear All-Natural Fibers - Sure that rayon dress FEELS like cotton, and that polyester shirt SEEMS airy, but synthetic fibers do not breathe. Stick (or un-stick, rather) to cotton, silk, and linen.
• Protect Your Hair Colour - Dyed hair needs sunscreen, too, especially if you have a bright colour like red (trust me, I know). Get a UV protective spray, like L'oreal's Ever Pure UV Spray, or get a hairspray with UV blockers like John Frieda's Anti-Frizz Hairspray.
• Take A Powder - Body powder, that is! As you get dressed after moisturizing, sunscreen-ing, and deodorizing seal it in and smooth yourself with baby powder or any corn starch-based powder. My fave is Silky Underwear by Lush; it smells heavenly of jasmine and has cocoa butter in it so I can ditch the lotion most days.
• Prime - If you wear makeup, especially liquid or cream makeup, make sure you put a primer down before you apply it. It will keep your makeup on your skin longer and will keep the makeup from clogging your pores. You should do this year round, but on hot days I notice my makeup breaks down faster in hot weather.
• Corn Starch Is a Miracle - If your hair is too frizzy to wash every day when the temperature and humidity rise just work some corn starch into your scalp. It's the active ingredient in most dry shampoos and only costs pennies. The only caveat is that you MUST work it in well or you will look like a judge of yore. Another great use for corn starch? Rub some on your bare feet before putting your shoes on- it will help wick the moisture, preventing discomfort and blisters. Seriously, for pretty much $1 for a decent sized can this is epically useful and will last a long time. I keep it in the bathroom.
• Carry Baby Wipes - This one I got from my friend Alma. On days you are really sweating, keep these in your bag for a quick touch-up throughout the day. It will cool you down; rub on back of your neck, knees, arms, underarms. If your deodorant isn't doing its job anymore use one of these to wipe it off and reapply; deodorant sticks best to clean, dry, recent shaved skin.
First two images found on Google, other two were found on Google image search.