If you do your makeup yourself, do your makeup how you usually wear it for an average day, or possibly a nice dinner where the steak is $30 but not $75. Do not wear heavy makeup; a close headshot will show clumps of mascara or dark lipstick. Choose light to medium eyeliners, eye shadows, and lipsticks that are only a shade or two darker than your skin tones. This brings your features out in a subtle manner without making it look like you're wearing lots of makeup. Avoid caking on heavy foundation, or wearing shiny eye shadows or lip gloss: too much shine is distracting on camera and looks wet.
Skin blemishes, pimples, and even wrinkles can be retouched. It’s not cheating. I promise.
How much is too much makeup for a headshot? Everyone is different, so it depends on how much make-up you usually wear: you want your headshots to be easily recognizable as how you generally look in person. For some people, that means wearing your make-up exactly how you usually wear it, for others it's wearing a little more, for others a little less. If you don't wear much makeup every day, don't do your makeup like Kim Kardashian for your photos.