Sienna Miller isn't the only one rocking the plaid shirt lately- opening hipster bible Vice's annual fashion issue shows page after page of plaid shirt devotees in countries all over the world. They're been work over dresses, worn as dresses (did Mary-Kate cause that?) and worn over leggings and paint-on jeans. How to adapt the look for warmer weather? Roll up the sleeves and throw it on as an eccentric cardigan. Want to remind folks that you have a waist? Just belt the whole mess. This is not a look for the trend-shy.

If you want a grungier, more authentic look, I'd recommend Goodwill, the men's section of a vintage store or any young mens' section of a low-end department store. Here's an okay (but $44) choice from the mens' department at Urban Outfitters. Why does this work? The Western shape will make you don't get lost in the cut- see how Sienna's look fits her at the shoulder?
If this look simply isn't for you, I've posted some more feminine but still offbeat takes on the plaid craze at the top. From top left: plaid shirt tunic $22.80, Forever 21, available here. Orange plaid dress from Echo Red by Mark Echo, $59.00, Macy's, available here. Plaid sloucher boot, $99.95 (orig. $248), Anthropologie, available here. Brixton plaid Hooligan Hat, Revolve Clothing, $36, available here.
By the way: plaid shorts are huge this year too, but I'm skipping that look because I don't feel like that preppy, pastel spin on the look- favored by the Pac Sun crowd- is part of the same trend.