How can I achieve Mary-Kate&Ashley's and Nicole Richie's style?

I absolutely love the boho-chic style they have, especially Mary Kate and Nicole Richie.Their looks seem effortless and thrown together on whim, and I would love to dress like them. The clothes at Urban Outfitters are ones that I like, so are the clothes there anything like what my idols wear? What can I do to achieve that style? Are their certain staples I need?