And here's Mischa Barton out at a recent event. She does up the makeup, but not her hair. (Check out her bangs, casually swept to the side—blow-drying your bangs straight in 90 percent humidity? Never recommended.) One key to pulling off the undone hair? Play up everything else—i.e. makeup, shoes, dress. Oh, and if your hair is falling seriously flat mid-way through the night, simply flip your head over, give your hair a shake with your fingertips and flip back. Instant volume.