Here are some important points.
1. Buy an unlocked phone or have yours unlocked.
2. If you are with Rogers you can switch to "Pay as you go" when you leave Canada If you don't have a contract. It costs $10 to switch and thats not monthly that's a one time fee. No more charges from Rogers while you are in the US if you don't use the phone and you keep your phone number.
3. When you get to the US go to AT&T and get a "pre paid" plan. The sim card is free. They will take out your Rogers sim (save it for the return home) and put in the AT&T. Watch now they do it so you can put your Rogers sim card in when you get back to Canada.
4. AT&T has a great plan for $60 with 8gb of data that rolls your unused data over to the next month. It's actually unlimited data that slows down after your 8 gb are used up. That's a lot of data in a month. You can use your phone as a "hot spot" with your computer. Other plans are available too.
The beauty here is your Rogers charges stop while you are in the US and the plans are cheaper in the US. This works well but you have to be away for a few months to make this worthwhile.