I think there are a few key rules to pulling off this look so you don't head straight down Frumpy Lane. First, pick a light, flowy fabric to give the blouse a feminine shape. If the top is too structured you run the risk of looking like a librarian. And second, pair it with a less conservative bottom. I'd choose a pair of skinny jeans or tuck it into a pair of dressy shorts or a pencil skirt with some fun heels.
My favorite that I've found so far is J. Crew's Secretary Blouse. But J. Crew can usually do no wrong in my eyes!