When we talked to New York City hairstylist and salon owner Eva Scrivo, she hadn't washed her hair in three days but looked as if she'd just had a blow-out. Here's her strategy:
Blow-dry hair with an ionic hair dryer and a round, boar-bristle brush. Don't use any styling products—they'll attract dirt from the air—and don't turn off the dryer until your hair is completely dry, or it will frizz. Before bed, tie your hair back very loosely—so that you won't create a crease—or not at all. Then sleep on a satin pillowcase, which will keep your hair from getting tangled.
If your hair looks a little flat in the morning, mist your part and hairline with water, and then use a blow-dryer and round brush to dry the front and top sections.
Repeat the Day 2 routine, but after you blow-dry, spray a dry shampoo on your part. Then repart your hair every two inches and spray a little more, aiming at the scalp each time and holding the can at arm's length. Use your fingers to work the dry shampoo all over your scalp, and then brush your hair.