Allow Yourself Enough Time
There's nothing that sets up a makeup disaster better than being in a rush and not really knowing what you are doing. Applying makeup is a skill, and as with any skills, it takes some practice to learn to do it well and quickly. You never know when something is going to go wrong, and when makeup goes wrong it might be something simple or something that requires starting over entirely. If you are going to try something new with makeup, or use a new product that is really different than whatever you've used before, do a makeup practice run ahead of time.
Start With a Clean Face
Makeup goes on best over clean skin. Washing your face about a half hour before you are going to start putting on makeup is recommended as then your skin can relax after the washing. This will not only let your skin tone return to its normal shades and tones (hot or warm water will change the appearance) but it allows the pores to close back down, which keeps makeup out of them. Don't do really harsh scrubs, as these can really open up the skin surface and allow the oils and chemicals of makeup to get into the pores, which leads to breakouts later.
Know What You Are Doing With Your Hair Before You Start Your Makeup
If you have to do hair styling as well as your makeup, know what you are going to do with it before you do any makeup. Be sure that anything you might do to your hair as far as washing or wetting it is done prior to doing makeup, as one wrong splash could ruin it. Some hairdos require curlers or putting the hair up in some way, and if all you'd need to do is take the hair down and quick style it, then you'll want to do that prior to your makeup. Also, make sure you have a way of keeping your hair out of your face while you do your makeup.