First, spritz your hair with a dry shampoo, we like Klorane Dry Shampoo with Oat Milk, or for girls with fine hair, try Oribé's Côte d'Azur Hair Refresher, as it doesn't weigh down light locks and smells delicious.
Then, pick up a simple black Alice band (or even a piece of black ribbon!), slip it on your head (you can part your hair in the center or pull it straight back) and then pull your remaining locks into a bun, ponytail, or, just like this look (which is from Valentino's A/W 2013 show), a braid. And voila! The top of your head, which is likely to look the greasiest, is hidden from sight, and you've got a style that works just as well in a corporate office as it does at the playground.