9. The Denim Jacket — Our favorite street-style heroines know how incredibly versatile a good denim jacket can be all year round, and this spring, it's the ultimate casual layering essential. Broken in and a tiny bit shrunken (this is the key!), the denim jacket works overtime, going from an after-hours party paired with some sequins to a long weekend walk through Central Park. The rule of thumb, right through to fall: Don't leave home without it.  

Burberry Leather Epaulette Denim Jacket, $450, available at Burberry; H&M Denim Jacket, $39.95, available at H&M.