The Canadian PR residency requirements are living in Canada for at least 2 years out of every 5 years.
For the US, I found this article about requirements there, http://www.visalaw.com/01jan4/12jan401.html which says that you should continue filing taxes there and keep a bank account. Back in the day, I knew some people who kept their green card for years by going to the US every 6 months but now after 9-11, I think they are watching their comings and goings more closely. Coming to the US every 6 months or every year is not guaranteed to keep you in PR status in the US anymore. If they believe you have moved away, they can still take it away. I would try to keep both and see if you can get away with it.