I am a Canadian citizen born in Canada. I work for myself as a consultant. There is a Canadian company that wants to hire me to visit a customer of theirs in the US. My work would be verbal consultation only, giving recommendations for ideas and improvements, instructions, work procedures etc.
Do I need any special visa to do this?
I will be paid in CDN$ by the Canadian company which is sending me there.
What is the best way to explain this at the the US Border crossing?
You are allowed to visit for "business" which is different from "work". You can also (if you have a bachelor's degree) get a TN1 for temporary work, where the US company will pay you. Visiting to discuss or finalize a contract is clearly business. Visiting to observe a pile of stuff and then going home to do the work of writing up all the instructions and recommendations would also be business. Actually delivering training could be either. It sounds like you're going for meetings, and that's what you should tell the officer.
Be cautious with advice to say little. While you can indeed say things that will get you (unfairly) refused, because you don't realize you're saying a word that means something special to the officer (eg work vs business), I had a staff member refused a TN1 he was eligible for because "you're just answering everything with one word answers and we think you're hiding something." Be confident and open with the description of the reason for your trip.
