Porteños are an appearance-conscious bunch who choose fashion over comfort any day. But though locals are stylish, they're usually fairly casual. With the exception of truly posh establishments, a dirty look is usually the only punishment restaurants give the underdressed; refusing entry is almost unheard of. A jacket and tie or stylish dress are only necessary if you plan on some seriously fine dining.
Your nicer jeans or khakis, capri pants, skirts, and dress shorts are perfect for urban sightseeing. Combine them with stylish walking shoes or leather flats; sneakers are fine if they're out-about-town and hip (leave the beat-up runners behind). In summer many local women seem to live in nice flip-flops or sandals. Trendiness takes the backseat in most smaller towns and villages, where dress is more practical and sometimes more conservative.
Wherever you go in the country, take good-quality sunglasses, sunblock, and a cap or hat: the sun can be strong, and you're close to the ozone hole, whatever sun-mad locals tell you. A good insect repellent is useful in Buenos Aires in summer and invaluable in Iguazú year-round.
For beach vacations, bring lightweight sweaters, a windbreaker, or a jacket for evenings when the temperatures drop. In winter temperatures rarely drop below freezing in the northern half of Argentina, including Buenos Aires, but a heavier coat or jacket is still a must. Temperatures drop dramatically at night in the high-altitude towns of the northwest, so bring a jacket even in summer.
Pharmacies in Buenos Aires stock a good range of toiletries, including some international brands, and hygiene products (note that only non-applicator tampons are available, however). Pharmacies, supermarkets, and kiosks sell condoms (preservativos), and oral contraceptive pills are available over the counter.
Toilet paper is a rare find in public restrooms, but you can buy pocket packs of tissues (known by their brand name, Carilinas) in kiosks. Antibacterial wipes and alcohol gel, available in pharmacies, can make bathroom trips more pleasant in remote areas. Don't forget to check camera batteries or chargers and bring along ample memory (or your favorite film).