How? With a little bit of effort. Spend 5 min. reading online reviews of the service before singing. Check the roaming rates before going to Canada, it takes 1 min. to find and read http://b2b.vzw.com/international/naroaming.htm
Here phone companies literally give someone unlimited credit to hang themselves. Add that to 45 day period before you know about the bill. Yeah, its stacked against you.