If you add just one piece of denim to your wardrobe for spring, make it a pair of cropped flare jeans. Mark our words, the kicky Audrey Hepburn-meets-Jane Birkin style is going to be everywhere this season. What to look for: straight-leg jeans in a medium or dark wash with a cropped hem that shows off your cute ankles. They look great with a blazer or simple sweater, but once the weather warms up, try them with a basic T-shirt. For shoes, pair them with booties or chunky pumps right now, and…