Summarize:

Don’t do anything you wouldn’t ordinarily do to your hair for a passport photo.  Passport photos should be a good representation of what you usually look like so that you don’t get detained. Do not wear a hat or any other head covering unless you already do so on a daily basis for religious purposes. If you do wear a head covering, for a U.S. passport, your face has to be visible. The covering can’t hide your hairline or cast shadows over any part of your face. If you usually wear makeup, go ahead and put it on as usual. If you never wear makeup, you probably don’t want to put on a ton of it for the photo. You won’t look like yourself, and you could get detained.  Wear a little oil-absorbing powder to prevent shine if you want to. This will be most useful on your forehead or nose. Even if you don’t usually wear makeup, you may want to put a bit of concealer or powder over any dark circles under your eyes. These dark places can cause a glare (and make you look sick or tired). Remember that you may need to use your passport for many occasions other than traveling.  (For example, sometimes you can use it to get a background check when you apply for a job.)  Try to wear solid colored clothing in muted tones.   Wear something flattering and reasonably comfortable. Don’t wear anything too flashy, or it will cause people to look at your outfit instead of your face. Pay most attention to your shirt, since that’s what will be in the photo. Scoop or v-necks work nicely.If it’s too low cut or if it’s a tank top, you might look naked, so check the neckline.   Since you will be in front of a white or black background, avoid those colors. Choose colors that flatter your skin tone. Wear a minimal amount of jewelry. Uniforms or anything that resembles a uniform (including camouflage) are not allowed unless they are religious attire that you wear every day.  Some people have reported that the passport office rejected their photo because it looked too much like previous photos (which means that the passport office couldn’t confirm that it was a recent picture), so you should dress a bit differently than you did in your last photo if this is an update.
Style your hair. Put on approximately the same amount of makeup as usual. Dress appropriately.