I don't know if it's the brutally hot and humid weather this August or if Hollywood's top hair stylists are all on vacay, but more and more actresses are pairing their high-toned dresses with low-key hair. Check out Renee Zellweger, above, on her way into the "Letterman" show last night: satin, blue fancy-pants dress, and low, loose pony. (And this isn't her first foray into super-casual hair, she's been doing it all week!) Personally I love this look—not only because I hate trying to tame my hair in 90-degree humidity, but also because it tones down the megawatt dress a little and makes the whole look less precious, like you don't take yourself that seriously. Which c'mon, you're wearing a YSL dress, do you really need the major up-do to get noticed? Check out these other stars who have fallen on the casual bandwagon—plus the products and tricks that keep their hair from slipping from pretty-messy to just plain messy territory. (Yeah, we know it's a fine line.)