Typical of the Southwest and southern Rockies, temperatures can vary considerably in North-Central New Mexico from sunup to sundown. Generally, you should pack for warm days and chilly nights from late spring through early fall, and for genuinely cold days and freezing nights in winter if you're headed to Taos and Santa Fe (Albuquerque runs about 10 to 15 degrees warmer). Because temperatures vary greatly even within this relatively compact area, it's important to check local weather conditions before you leave home and pack accordingly. In April for instance, you may need to pack for nighttime lows in the 20s and daytime highs in the 60s in Taos, but daytime highs in the low 80s and nighttime lows in the 40s in Albuquerque. Any time of year pack at least a few warm outfits, gloves, a hat and a jacket; in winter pack very warm clothes—coats, parkas, and whatever else your body's thermostat and your ultimate destination dictate. Sweaters and jackets are also needed in summer at higher elevations, because though days are warm, nights can dip well below 50°F. And bring comfortable shoes; you're likely to be doing a lot of walking.
New Mexico is one of the most informal and laid-back areas of the country, which for many is part of its appeal. Probably no more than three or four restaurants in the entire state enforce a dress code, even for dinner, though men are likely to feel more comfortable wearing a jacket or at least a sport shirt in high-end restaurants in Santa Fe, especially during the summer opera season.
Bring skin moisturizer; even people who rarely need this elsewhere in the country can suffer from dry and itchy skin in New Mexico. Sunscreen is a necessity. And bring sunglasses to protect your eyes from the glare of lakes or ski slopes, not to mention the brightness present everywhere. High altitude can cause headaches and dizziness, so at a minimum drink at least half your body weight in ounces in water (150-pound person=75 ounces of water), and eat plenty of juicy fruit. When planning even a short day trip, especially if there's hiking or exercise involved, always pack a bottle or two of water—it's very easy to become dehydrated in New Mexico. Check with your doctor about medication to alleviate symptoms.