1. Smiling
2. (properly done) Makeup
Ever watch those "before and after" pictures (that show the face), which usually show how bad then good one looks after a certain change? Usually for weight loss programs, make up, or skin remedy. The "before" picture usually have the people looking so sad and miserable about their state. Then the "after" picture they are smiling and happy, and thus look as if they have improved [and usually wearing make up!!! weight loss program commercials have the people "tanned" in their after shots, and acne med commercials may have ONE non-makeup photo, and everything else they're full-faced made up!!]. Smiling totally makes a difference.
And make up is meant to enhance the beautiful parts of the face, and disguise the not-so-beautiful parts. (side note: I sometimes like to use make up as a form of self expression, just like the style of clothes one chooses to wear. i can do "pretty" make up when I want and need to do it). eyeliner and mascara usually adds definition and interest to the eyes. eyeshadow and blush usually create the illusion of face shape and dimension. lipstick shows sign of life and health. concealer and foundation mask the imperfections.
there are plenty of pictures of "gorgeous celebrities without make up and thus look crappy," but to keep up with current events, I shall discuss Kate Middleton. Everyone comments on how beautiful she is. Do I think she is beautiful? Yes. But I can't help but wonder "What does she look like without all that black eyeliner and mascara framing those blue eyes?" So I Google "kate middleton no makeup." Some results were web articles stating "Kate is such a natural beauty, as she is often seen without make up," but 90% of the pictures they provide, she IS wearing make up. She may not be wearing foundation, and with my microscopically scanning eyes, I can see some imperfections, but she is clearly wearing black eyeliner and mascara.
the remaining 10% is indeterminable because it is not a frontal close up, poor lighting, sunglasses, etc...
this is the only picture I've found that she is POSSIBLY not wearing make up, since she is participating in an activity where make up is not necessary. there is still a possibility that she is wearing make up in this photo. anyway, back to my point, this is a poorly shot photo for determining whether or not she is wearing make up. But she looks great, because she is smiling and looks happy and enjoying herself.
So, if you want to feel and look pretty, smile. The make up is optional, too.