Key Words
Ask them. Can I "try out" my metal detector in your yard, lot, etc. These words are very effective, they are non confrontational. Always step way back after you knock on the door. If the answer is no, thank them and then say that I did not want to trespass on anyone's property. If you get permission, share any clad, pennies, nickels etc. with the kids if any are present. If it is an older person, and they watch you from the porch, take a break, set on the steps, and talk about the weather, gardening, the past. You will be surprised how many other doors, and places this will open. Show them what you have found, (within reason) and offer to share. You can leverage this permission with the neighbors. "Mr. Jones let my try out my detector in his yard, may I also try it in yours." If the person asks you to locate a property pin, well cap, lost tool, stop right then and look for what they ask. You will be able to come back another day if you run out of time. I used Tabman's technique of giving out tomato seedlings. You will be surprised how many properties this will open.
Hope these points will give you success. I have had very few people deny me permission in 7 years with this method. Hope you find a gold coin!
SGuardez in Southeast Missouri.