I can help you on this as I live between the two countries and have done this on several occasions. It's harder to bring a vehicle into the U.S. than it is into Canada. There are state and federal laws that come into play. Emissions being the big one. First and foremost go to the U.S. DOT site and make sure the year, make and model (and in some cases the serial number sequence) is admissible into the U.S. Some vehicles don't meet safety requirements or emissions requirements. The next thing find out what ownership documents your local DMV requires. In Canada there are no pink slips. Each Province has different laws when it comes to ownership (registration). Next and the most important thing of all find out if the vehicle has any debt or liens associated with it. If it's a new vehicle have the dealer provide you with a NVIS certificate and a recall clearance letter. As for duties if the vehicle is manufactured in the US or Canada and you have documentation to prove it (VIN plate on the drivers door will indicate that) there is no duty. The NAFTA agreement excludes duties. You will have to pay federal and state taxes; based on what state you live in. With the Canadian dollar being as low as it is it is a good time to buy vehicles from Canada to the US as there is an automatic savings of 25%; providing the price is right!

Once you do this the first time it's a little unnerving. If anybody tells you it's complicated they are incorrect. Do your homework.
My advice is to Google the importing a vehicle from Canada to the US and there's a lot a good sites that will have a checklist for you.