Using a makeup primer on your skin helps create a smooth surface for easier makeup application, but most makeup primers are either very expensive or loaded with chemicals. This is why I switched to using aloe vera gel as a makeup primer instead. It works just as well as most store-bought makeup primers and contains fewer chemicals that could possibly be bad for my skin or toxic to my body. It also work as a fantastic barrier against outside elements, such as pollution, which means healthier skin.
Not only does aloe vera gel fill in the small wrinkles and pores in your skin to create a much smoother surface for makeup application, I've found it is also very affective in controlling oily skin and shine. As a result, makeup looks a lot more even and lasts longer throughout the day. Aloe vera also has antibacterial and anti-inflammatory properties, and those characteristics combined with its ability to control sebum production makes it a great primer for those with oily or acne-prone skin.
To use aloe vera gel as a primer, simply spread a thin layer over your face as you would a regular makeup primer. I find a few large drops is enough. Wait a minute for the aloe to dry before applying your makeup. It may feel a little tacky on your skin, but makeup still goes on pretty smoothly.
Of course, unless you're using aloe vera squeezed directly from the plant, most aloe vera gels are not completely chemical-free, and will contain a few preservatives to keep it from going rancid too quickly. In this case, do some research and try to find a brand that has as few extra additives as possible, or at least chemicals that aren't known to cause irritation or other adverse side effects.
My aloe vera gel of choice is Fruit of the Earth Aloe Vera 100% Gel, 24 oz (680 g), which does contain a few preservatives, but nothing unnecessary such as added fragrance or coloring.