You can buy most anything you need in the state, even in the village outposts (though this does not include spare parts for your laptop), but one thing you don’t want to be caught without is mosquito repellent. Trust me. A face net for blackflies and skeeters wouldn’t be a bad idea if you plan on delving into the woods (or even camping).
Regarding clothing, Wisconsin is a place where most consider L. L. Bean dress-up clothing. Heels, ties, and fancy skirts are fine for clubbing in Milwaukee [1] or Madison [2], but you’ll be absolutely conspicuous in all but the most chichi restaurants anywhere else (hey, Badger sweatshirts are perfectly fine in even the most famous supper clubs).
Weather is often the deciding factor. Dress appropriately for the weather at all times—that includes wearing a hat. Do not come to Wisconsin in winter without a good pair of gloves or mittens. Inuit-worthy mittens are something you’ll be ever so grateful for on a sleigh ride or while you await a tow truck. A good pair of boots is also a necessity; some people carry a heavy-duty pair in the car at all times, in case of an emergency.
Always remember the mantra near Lake Michigan: cooler near the lake. This means it may be warm in the sun during the day, but once the sun goes down the temperature might drop precipitously compared to towns inland. (Thus, it is never a bad idea to have a pocket-sized windbreaker handy, even in summer.)
Given the state’s somewhat iffy weather, it’s paramount to prepare your car for any possibility by winterizing your vehicle.
Carry an emergency kit with booster cables, sand or gravel (in a pinch, try sandpaper strips or your car’s floor mats), flares, candles, matches, a shovel and scraper, flashlight and extra batteries, blankets (space blankets are excellent), extra heavy clothing, high-calorie nonperishable food, and anything else you might need if you have to spend the night in a snowbank. I cannot emphasize how important it is—I’ve stumbled upon out-of-staters stuck in a snowy ditch and been amazed that, had someone not stumbled along, they were woefully unprepared to spend a winter night in the car.
Links:
[1] http://www.moon.com/destinations/wisconsin/milwaukee
[2] http://www.moon.com/destinations/wisconsin/madison