Why would anyone want to use a makeup primer? Well, it helps your makeup go on smoother, lends staying power, absorbs oil, and can even help even out blemishes or uneven spots on your face. Some are really expensive and some are midrange. However, the least expensive and one of the best may very well be in your medicine cabinet. It doesn't even have to be name brand, as generic works just as well. Milk of magnesia (MOM) makes an excellent makeup primer and one small bottle can last for ages.
I always dilute MOM slightly by applying it with a wet cotton ball. The way you will use it depends on your skin type. If you have dry skin, you will want to apply a moisturizer with sunscreen first and then apply the MOM over the top. If you have oily skin, like me, you may want to apply it all over, and if you have oily skin in certain spots, you might want to just use it for those places.
Apply the MOM then allow to dry. If you have white splotches, just use the dampened cotton ball to swipe a little away. Make sure your face is dry, then apply your foundation and makeup as usual. I even carefully apply it to my eyelids, as it makes a great base/primer for eye shadow.
I tend to have oily skin. Oftentimes, if I don't wear some type of oil absorbing something or other, my makeup will have practically faded away by the end of the day. I use the MOM for makeup staying power.
How often to use this is up to you. Some people have said it has actually helped their complexion by absorbing oil and smoothing skin tone. I have also seen some people say that if they use this and wear it for a very long time it can cause breakouts, so try it to see how it works for you.
My personal experience has been that it provides an excellent base for makeup, allowing it to go on smoother. It's helped my oily skin problem, and my makeup has been fresh and pretty at the end of the day. It's easy to obtain, inexpensive, and it can help provide flawless makeup application.