The best thing about ATVing is that it is a sport you can do anytime of the year, but you must dress for the climate in which you are riding. If you are going to be riding in a hot climate, dress accordingly and if you're going to be riding in a colder climate and especially during Winter, dress appropriately.
There are a lot of accessories you can add to your ATV to make it more Wintertime riding friendly such as handlebar mittens, heated seats, windshields, and more,
The main thing you want to concentrate on when you're riding your ATV in the Winter is stay warm and dry. Those two go hand in hand.
The best way to stay warm in cold weather is to layer your clothing. In a day, you may encounter a wide variety of ATV riding conditions and terrain. Layering is the smart way to meet all these circumstances. You will need heavier clothing for easy riding ATV trails and lighter, less constrictive clothing for harder to maneuver ATV trails.
If you break into a sweat, remove a layer of clothing so you don't get overheated. If you wear cold, wet clothing in sub-zero temperatures, it's going to make for a really bad day of ATVing for you. Thermal underwear is a big plus in this kind of weather. Make it the start for any day you're going to go riding your ATV in cold weather.
Be smart in your layering, make the first layer your thermal underwear. Make your next layer cotton to absorb any moisture. After that, use fleece, down or fiber-filled wool, etc., depending on the outside temperature and your personal preference.