What to bring — Oaxaca
Most travelers dress casually in Oaxaca. Few, if any, places ordinary travelers visit require dressing up, unless they want to for an evening out. Besides, when traveling, anything expensive looking is a magnet for pickpockets and other ne'er-do-wells.
Except for the odd young Oaxacan surfer dude living the life of the beach bum, Oaxacan men and women never wear shorts, even on the coast and the tropical Isthmus. In beach towns, women travelers can wear short pants, skirts, and dresses anywhere; men travelers, shorts. Swimsuits, however, should be confined to the beach. Away from the beaches, men and women travelers sometimes wear shorts. This is tolerated, but out of cultural respect, should not be done. In all, Oaxacans are tolerant of the way foreigners dress.
The coast is warm during the fall and the winter and hot in the spring and the summer, so make sure to dress appropriately. Wear short sleeve shirts and blouses, skirts, dresses, and shorts made of light cotton and synthetic materials that breathe, as well as flip flops and sandals. Moreover, only bring clothes that can be hand washed in the bathroom sink and will dry overnight, eliminating the hassle of sending clothes out to be cleaned or, heaven forbid, schlepping down to the laundromat. In the evenings in the winter and fall, a light sweater may be necessary.
The city of Oaxaca and its surrounding valleys and sierras are another matter. During the summer and spring, evenings and early mornings can be cool and a light sweater is often necessary, while days are usually warm and pleasant and short sleeves are fine. In the fall and winter, however, nights can be cold, especially in the sierras, so make sure to bring a jacket of some kind. (The coldest this writer has experienced so far was a low of 40°F/4°C, one blustery late-December night in the city of Oaxaca. Mercifully, a warm fleece jacket was at hand.)
The article "What to bring" has additional thoughts on packing and luggage, as well as a handy checklist.