Pack lots of layers

Layer, even if you think you won’t need to. Forget what you’ve read about sunny California: San Francisco in summer is subject to finger-numbing fogs, and even Los Angeles has its chilly days. Also, malls, museums, cinemas, and airlines cabins are often air-conditioned to a wintry degree. The best bet is a lightweight, waterproof jacket, stuffable into your carry-on. A shawl-size scarf–virtually weightless and warm–is a woman’s best friend, and can act as a beach cover-up or wrap.

From the Forums: Bare minimum packing list 8 days in France in September?