Just start work.
I did used to knock when i started out, but that just seemed to reinforce how grateful i was for the work. I eventually stopped knocking and got rid of jobs that had gotten used to it and didn't it to change.
The time i wait for someone to come to the door over the day could be another job or 2 done.
And another thing, when i did used to knock, most still took ages faffing about finding the money when i knocked again after finishing.