I usually have a write up about myself and any type of hunting I have done (I did a talk at a public Library and the paper did a small story on it), so I use that picture and story. Giving them a sheet of paper with all of the information you want out in the public, says a lot about yourself. I never give out my address, but my name, town, e mail, even phone and that makes them feel a bit more comfortable. Also bringing some tidbits like a flat button, newer bullet, part of a buckle and maybe a wheaty always helps. Stress that you love history and this is your day off hobby (or full time hobby if retired). Always look neat and only approach if you see someone in the yard. It always helps if you know someone who knows the people you are looking to ask permission from. It could be a neighbor, friend, relative, etc... just some way of getting a warm introduction instead of a cold call. And last but not least offer to take all trash, do an excellent job of recovering and replacing the dirt and for quality places like the one you described, offer 20 bucks as a thanks for allowing permission. These are some of my suggestions, but I bet others have some great ones too.