Its a numbers game as everyone has said, nothing at all to be frightened of.
Just remember that every yes is income for a long time, I recently treated myself to a new phone on pay monthly, I feel good with the phone and one new customer pays for it.
The best way to approach the door knocking is to say something along the lines of ' hi, my name is ... of ...cleaning. Do you have a window cleaner at present?
If they say yes then I thank them and move on, if not then I briefly explain my service, cost, frequency etc and a lot of times sign them up.
Remember the more doors you knock the richer you will be.
Good luck