The right size: A purse that's approximately the same length and width as a file folder but not so boxy that it interferes with how your arm swings when you walk. Make sure it has an adjustable strap—a puffy winter jacket can hike up a bag that fits comfortably over a tank top in the summer, says Glassman.
The most common mistake: Choosing a smooth, buttery leather (like lambskin) that shows nicks and scratches, and sags under the weight of a heavy wallet.
One to try: Emerald green accessories showed up at shows like Stella McCartney and Mulberry. Choose a bag in this color to punch up a navy suit or a black-and-white print dress.