Messy hair is apparently hot right now (likely as part of a backlash against the super-structured pieces of last fall) and celebrities can't get enough of it. Blame it on their L.A. locale, summer laziness, or trend-centric stylists; regardless of the reason, slept-in hair is sticking around for awhile.
Stylelist (AOL's style blog) posted an entry about which celebrity looks hottest with the messy look. Shakira and Kate Bosworth pull it off pretty accessibly; Shakira's curly hair looks almost dreadlocked, it's so textured, while Bosworth's fine hair looks lovingly tousled. Mary-Kate Olsen, famous for her messy boho style, also makes the list for her roughed up buns and carefree tendrils.
If you want to try the look out yourself, start at the beginning. A cut without layers will likely end up looking pretty blunt if you mess it up, and you're shooting for carefree fuzziness, not trapezoidal frizziness. The right products will do all the work; steer clear of the hairdryer (except for your bangs, if you so choose) and apply a sea-salt spray; Bumble and Bumble's Surf Spray is an industry standard. (For something a little less pricey, try out Garnier's Wonder Waves collection.) Scrunch and twist your damp hair then let it air dry. Voila! Messily chic tresses!