Please look at the following document from Revenue Canada:
T4056 - Emigrants and Income Tax - 2007
I would suggest not trying to get a non-resident status through a lawyer. I tried this way and was denied....This is a very unpleasant uphill battle with Revenue Canada (also expensive).
Just follow the guidelines and you will have no problem. I have become non-resident from Canada twice. The first time was very difficult, for the reasons given above. The second time was a piece of cake.
I hope this help.