I don't bother with mass door knocking. I do enough walking with my MD, I'm not about to do more without it.
Networking seems to work for me. After we made Digging History with a website and some business cards, getting access to places got a lot easier. I carry the cards with me when I'm out hunting parks and whatnot, and people interested enough to ask if I've found anything get a smile, some brief conversation about typical finds, and a card with the suggestion that they give me a call if they're curious what might be in their yard.
Something else I do is associate myself with the local history society. I let the council know that I'm an MD hobbyist and I'd love to find stuff for their local history collection. Believe it or not, you will find more stuff you can keep for yourself than you would ostensibly donate to the society. This bolsters your credibility for door knocking, because instead of "Hi, I'm Joe Schmoe and I was wondering if I can metal detect in your yard" you can say something like "Good afternoon, my name is Jason Jones with the Local History Society, we have highlighted your property in our research to potentially contain artifacts of historic significance to our area, and I am here to request permission to do some surveying to see if any artifacts may be recovered." People appreciate professionalism, even if it's with a shovelful of bullshit.
In regards to public areas with vague regulations about impacts to wildlife, if someone calls the cops, the investigating officer will also receive a business card and a brief statement about what I hope to accomplish by MDing there. If you give the impression you intend to donate even just one thing out of the pile you dig, you can justify your presence and unless the guy is a real dick he'll likely leave you alone. I try to incorporate a short history lesson into these conversations because it makes it seem like you know your stuff. Usually the "lesson" is on the history of the area, or of something I found there recently. If nothing comes to mind and you're grasping at snippets for conversation, offer to allow the officer to try your MD himself.
