Hi there,
I am definitely not an expert of a lawyer but I am a Canadian. My knowledge is that Canadians are allowed to visit the USA, without a visa for UP TO six months at a time and vice versa. I have no knowledge of there being a restriction of the six month visit being only once per year, if anyone knows otherwise please advise.
From what I know other people have done in the past, you can literally cross the border back into Canada one day and then cross over into the USA the next and your six month term renews intself.
Also up until this year you could travel to the USA without even a passport. If you didn't know, as of January 23rd (I think..date could be off)any travel to the USA (even from Canada) by air or sea requires a passport. By land you can still enter the US with a birth certficate and drivers license.
I received this information from both the Canadian Immigration services and the USA over telephone and the internet.