In CIC's eyes it is okay for you to live with your partner - but in CBSA's eyes, you cannot "live" in Canada and you most certainly cannot "move" to Canada.
Just make sure you avoid using the words "live" or "move" - say you're visiting your partner and staying with him/her. Tell them you are visiting for 6 months (having a return ticket if you are flying is not a bad idea - they may or may not ask to see it, but it's good to have one. You can get a fully refundable one way flight back, or you can buy a return ticket and not use the return portion.) and if they ask about your future plans, say that if you decide to stay longer, you will apply to have your visitor status extended.
Bringing your clothes and whatnot are fine, but if it's going to exceed a suitcase or two, leave stuff at home and/or have your family send you a parcel later on. When I arrived in March, the customs agent asked what was in my two suitcases - I truthfully enumerated the stuff I had - clothes, shoes, scarves, makeup, hair dryer, etc. He asked if any of it is staying in Canada - I said no, it's just personal items and they go with me. He also casually asked if I was going to do the "six months thing" - stay for 6 months, cross the border and come back for another 6 months - and I said I'd already applied for PR and I don't know how long that's going to take, so I'll later decide if I want to go back to Japan to wait the PR out or apply for an extension. They'll be satisfied if they feel that you know the rules and that you're not going to try to circumvent them or anything.
CIC on the other hand won't raise an eyebrow if you change your mailing address to Canada. You can do it after you're in Canada, through e-cas.
