I always knock the door and say
Hi, sorry to bother you but just enquiring to see if you have a window cleaner?
They say either, yes, no or how much, if they say say no I ask if they are interested or somwetimes they have said yes and I then reply no worries, and thanks for your time and walk off, but sometimes before I get to the bottom of the garden they are asking how much do I charge then you can go into what your company offers and how regular the service will be.
I do have good success with canvassing as people say they dont like going out getting it but once you get that first" YES" then all the "no thanks" you just forget about