Proof of living together for common law application

My girlfriend (sponsor) is 19 and a student living with her parents in their place down in the basement.
I moved in with her in late October 2008 but because its her parents house they never gave me a lease or anything like that to sign because they didnt have too.

Now the only proof I have that I have lived there from late october 08 was my Cell Phone Contract that I have under that address that I signed up for as soon as I moved in there.

What should I do to prove that we lived down there together?
We never had bills because we just split it with her parents under their name
We dont have a joint bank account cuz we just took turns buying groceries etc

We only have cell Phone bills in my name at that addess from October and then I bought a car in Jan 09 and registered it under that address. I know those can be used as proof that we lived there together but its probably not enough (we do have countless other letters to us at the same address)

could I get her parents to write a letter saying that me and their daughter have been living together in the basement of our house from October 08? and would someone need to sign it or would thier signatures be ok?

*** we will be applying in November 09 for the commonlaw immigration*