When canvassing, just remember the 4 P's. Be polite, professional, positive and persistent. You may be nervous, which is natural, but remember the householder may be as well, so the onus is on you to put them at ease. Smile and try to convey the feeling that you are relaxed. Also, try to treat each door as if it is the first.
Whenever I canvass, I try to be all of the above. I say something along the lines of 'Hello, my name is John, and I am offering a window cleaning service in the neighbourhood, and am wondering if you would be interested at all?'
You may get 100 rejections with this approach (I knocked thousands of doors but now have 300 odd customers) but you will have success if you keep trying. Remember that every 'no' that you get is bringing you closer to the next 'yes'.
It is hard going, but if you are determined and self-motivated enough, you will get there in the end.
Best wishes,
John