In New York, especially in New York City and the surrounding suburbs, jackets and ties are required for men at a few high-end restaurants; in general, New Yorkers tend to dress a bit more formally than their Midwest or West Coast counterparts for special events. Jeans and sneakers are acceptable for casual dining and sightseeing just about anywhere in the state. Be sure to pack sneakers or other walking shoes for pounding the pavement in urban areas, and hiking boots if you plan to hit the trails.
In spring and fall, pack at least one warm jacket and sweater, since moderate daytime temperatures can drop after nightfall. Bring shorts for summer, which can be quite humid. You'll need a warm coat, hat, scarf, and gloves in winter; boots for often slushy streets are also a good idea. Upstate New York has some of the coldest weather in the eastern United States, so pack particularly warm gear if you're spending time in the state's northern and western regions in winter.
Pack light—porters and luggage trolleys can be hard to find at New York airports. And bring a fistful of quarters for renting a cart.