V-Man,
I don't think I would call it a pitch line, like you are selling yourself. Though I get what you mean. I've always thought of it as "How can I make friends with the person on the other side of that door threshold in the next 20 seconds."
I've been knocking on doors cold turkey for more that 25 years. Here's what I do MOST of the time. The rest of the time, I may have to improvise and do the best I can and remember to be as nice and friendly as possible.
I ring the bell and then stand back from the door at least 3 or 4 feet so that whoever answers the door doesn't see me right up in their face. I am 6'3" and am taller than most and don't want to make the person feel intimidated or defensive in any way. If I am wearing sun glasses, I take them off, so that they can see my eyes. I put a smile on my face

and some friendly in my voice

even if I've spent the last three hours digging pull tabs. If I've had a bad day, that's my problem, not theirs.
When the person answers the door,
I say, Hello, my name is Rich (Utah), as I was driving by, I noticed that you live in a very old home. I have a hobby of looking for old coins and trade tokens in the yards of old homes like yours with my COIN detector and was wondering if it would be ok to search the yard?
(I do not use the words METAL DETECTOR)
Depending on what the yard looks like, I may ask to look only in the front yard. On occasion, I may only ask to search the parking strip out in the front of the property. (public property around my parts, but still maintained by the homeowner) It depends on my gut feeling of the person at the door the moment it is answered. If we establish a good rapport at the door I may ask if they know any of the history of the house? Has it always been in their family? and so forth.
If I've already been hunting other houses in the neighborhood, I go up to the door with all my gear on. I do make sure that my Lesche digger is sheathed and back behind my hip, again, I don't want anyone feeling intimidated or threatened. I've often gotten into conversations about my equipment and the type of stuff I usually find. Again, more chance to talk and make a friend.
Earlier this year, a lady that answered the door was right in the middle of getting dinner ready and I had truly inconvenienced her, yet when I asked her about the history or her home, she paused and started sharing with me the story about their house. It was quite interesting. Take the time to talk to people, and to listen as well.
http://www.findmall.com/read.php?63,1229725,1229725#msg-1229725
When I finished I thanked the lady and gave her the 1927 Wheatie that I had found in the parking strip. I have jars of wheaties and won't miss it and she was genuinely glad to receive it as a little token of appreciation.
Having done this for decades, you will meet all sorts of wonderful people. Remember the greats ones and enjoy being out there.
The last few years I've been having better luck with older owners. It might be that my hair is greying. Anyway, older people like to talk more about the histories of their homes and in general seem friendlier.
What about the younger folks buying up these older houses? Some of the newer generation wants to know what is in it for them? A fair question I guess. I ask if the house has been in the family for any significant length of time and if there is a possibility that any family heirlooms or such may have been lost? If so, I offer to look for a lost ring or whatever while I am looking for coins and such and will happily return it.
On occasion, they want to know if I am looking for a treasure of some sort. And while I guess that we are looking for little treasures, most of them aren't that valuable to be concerned about, with rare exception. I've asked them if they know of any treasure that we should be looking for? Most likely they don't. If they want half of anything significant, like a cache of coins or something, depending on my mood, I usually accept. If they want half of everything that I find, I usually move on. These are often the folks that hawk over you as you hunt through the yard and make the hunt less than enjoyable.
If they say yes, well, away you go.
Some aren't very comfortable doing this, and that is fine. It does take a certain disposition to knock on doors and it may not be your cup of tea. If not, hook up with a friend that doesn't mind. You can do the research on possibilities and they can do the asking.
For years, I've hunted with a friend of mine, Sid, who's is as friendly as they come. When we've gone hunting, it is a true adventure. We would find a few old houses that looked promising and start knocking on doors. Whoever got permission to hunt the yard has first pick as to which side of the yard to start on. We typically divided the yard in half, with the front walkway being the border. When we finished, we would switch sides and see what each other missed. (that's a whole other story) Thing is, this method worked for us, we had fun, made lots of finds, became VERY good at picking out the best side of the yard to start with, and made the most of our time.
Well, that's about it.
A few tips, I would make sure that you are able to retrieve a deep coin in a manicured lawn before you try it in somebodies yard. Make sure that you clean up after every recovery as best you can, and if you aren't very good at it, practice in your own yard and see what works and what doesn't.
Remember:
1. Shallow plugs die and go brown.

2. If you take a deep plug on a 3" coin and take all the dirt off the roots of the grass looking for said 3" coin, the plug will die and go brown.

3. Tuck in the edges of your plugs for a cleaner look and to keep the edges of the plug from going brown.
4. Fluff up the grass BEFORE you cut the plug with your digger to prevent giving the plug a 'haircut' and making it more obvious where you dug.
5. If you remove any soil from the hole, use a ground cloth. It will prevent 'staining' the adjacent grass with dirt. It will make it easier to recover a target from the removed dirt. And it will make it easier to get all of the dirt back in the hole when you are done. (more on this)
6. If you are digging for deep coins in compacted dirt, you may end up with more dirt once it has been removed and broken up than will fit back in the hole. You do not want a cut plug sitting above the level of the rest of the grass. It will look unsightly to a homeowner. A mower may pull it out and most likely the edges will die leaving a brown ring around your plug. If you are wise, and have been using a ground cloth like you should, you can take the excess dirt to a garden area where it can be sprinkled inconspicuously. There have been times I've carried a large amount of excess dirt in my trash apron until I could get rid of it.
7. Treat the yard like it is your own. If you don't treat your own yard very well, treat it like you would if you were hunting the lawns of Buckingham Palace and if you left a mess, the Queen would have you flogged.
I am sure there are other ways that work as well. I am always happy to learn another way to do something.
Best of luck,
Rich (Utah)