In general, New Englanders dress casually. Even in Boston , jeans are fine in all but the most upscale restaurants and hotels. When in doubt, anything from the L.L.Bean catalog will do.
Sweaters and heavy coats are a must in winter—supplemented with long underwear and wool socks if you plan on being comfortable during long stints outdoors. A light jacket and several layers are often the best choice in spring and fall, when temperatures can change dramatically over the course of a day. T-shirts and shorts are standard in summer, but even in August be sure to pack a light jacket or sweater, as nights can sometimes be quite cool, especially on the coast. There is only one item of clothing that you wear at your peril: a New York Yankees cap!
Invest in a good road-map book if you are going to be doing a lot of driving in rural areas, where roads can be confusing and locals less than helpful. (The classic joke in Maine: A man stops to ask directions. “Can you take this road to Bangor?” Local responds (in a thick Maine accent): “Ayuh, but I think they already got some theah.”)
Sunscreen is recommended in summer for those with fair skin who don’t want to look as red as the lobster they’re eating. Bug repellent is also essential when visiting for outdoor areas, especially during black fly season in early summer in New Hampshire  and Maine —not that it will help, but it’s good to know that at least you are doing everything you can.