This is where a leaflet drop first comes in handy. As they open the door you say "Hello, my name is ........... and I put one of these through your door a couple of days ago" and show them the leaflet. It's still technically cold calling, but at least if you have a uniform with your name/logo on it and your leaflet also has your name/logo you've got a sort of 'prior introduction'.
As for those yellow 'no caller' stickers, I always knock, then when they open the door I say: "I'm sorry, I knocked before I noticed your sticker, then it seemed rude to just walk away without explaining." Gets them every time, they just say: "Oh, that's alright!"