Steve, the way I go about it is like this. I drop leaflets saying who I am and how long I've been in business. What service I provide, (6 weekly) and that we're reliable and friendly etc. I also tell them on the flyer that we'll be calling in the near future to see if they re interested etc. About 3 or 4 days later, I then go door knocking. Works well for me. Another way is to pay some guy like Mick Kent to do it for you. From what I've seen on here, he seems to know what he's doing. It takes bottle to do what you've done. Full respect. Just remember, some will, some won't, so what, nock on the next door.
The difference between guys with lots of work and guys who have problems getting work is who knocks more doors. You grow your business when others would give up and go home. It sounds like you're that sort of bloke. Keep going.