Canadian Contractor regularly visiting US

Hi. I am a Canadian citizen, working for a US company remotely from Canada as a contractor (not an employee). I've already figured out that I only pay taxes to Canada (thanks to this website!). My question at this point is about travelling to the US.

I would like to know what I should or need to be doing or getting, as far as TN or H1 goes, or anything like that. My intent is to travel to my US client and actually do work on-site for their company. Say, 2 to 4 visits a year, each visit being about 5 working days long (up to 7 days, if you count landing and taking off, but not actually working). I've read a bit about the Canada-US tax treaty, and that this falls well below the 183 day limit, and also below the $10000 limit.

I've also heard about NEXUS -- any advice regarding that?

How can I be best prepared to respond to any questions at the border regarding my purposes in the US? Any particular wording to use or avoid?

Thanks for any and all responses!