You manner, and looks play a part.
If you dress like a scruff rough, do you really expect to gain custom? (no offence to anyone that does - but in my eyes you are a public serviceman, and therefore should wear a company uniform) I hate letting any gas engineer in without a uniform of the company, i always ask for ID and will then check it if they are not in company uniform.
Be apologetic to them when you knock on their door (sorry to disturb you.... yada yada..)
Get a CRB check done - its helps to inform of your background.
I find dropping a well presented flyer through the door and then knock on the door a few evenings later and say (with a copy of the flyer in hand) " i dropped this leaflet through the door, have you had a chance to see it? what did you think?, Would you like a free quotation whilst im here, and then maybe i can be of service to you now or in the future?
If it works, then great if not, try a few months down the line..
Think about it, what would you like to see if someone was trying to sell you something on your doorstep? Would you prefer to have seen them around a bit first?
Getting good custys is about being known.