The ultimate key is wearing less makeup. During the summer I use 4 products, instead of 10-15. BB cream, all in one color, waterproof mascara, and setting spray are all that is needed to look great and drip free in the summer.
BB Cream is one liquid product that blurs imperfections, brightens the face, evens skin tone, moisturizes, smoothes the skin, enhances natural beauty, acts as a sunscreen, and is oil free. My personal favorite is the Maybelline Dream Fresh BB Cream that can be found at Ulta and most drug stores for around $8.
All in one color is one product that acts as a lipstick, blush, and eye shadow. It brightens the everyday makeup look without having to wear several different products. The simple tint adds a beautiful, radiant glow. My favorite is Stila's Convertabile Color, which retails for $25 at Sephora. It comes in six different shades, but my favorite is Petunia (featured below.)
Waterproof mascara is a given for any summer day, especially when you will be at the pool or the beach. There is no worry of having raccoon eyes or black streaks down your cheeks when you wear it. I wear it year round because I am a big softy who cries during commercials, but I even recommend it in the summer over false eyelashes. My favorite is Maybelline Volum' Express The Falsies Flared Waterproof Mascara in Very Black. It can be purchased at Ulta or any drugstore for about $7.
Setting spray keeps your makeup from moving. It can be 105 and humid, but your makeup is going no where. I always finish my makeup application with setting spray. My favorite is the Urban Decay All Nighter Long Lasting Makeup Setting Spray. It can be purchased at Sephora for $14 and holds your makeup for 16+ hours.