Research the place. You want to be able to express your interest to the owner with facts, like the old school's name and dates of operation. Also research the owner, call him by name (if it is a women, just forget about it, and go get a few beers.. JK), some people like to door knock, I prefer to call and ask them if I can come by introduce myself, and promise not to take more than 5 minutes of their time... if it goes well, they will want to talk with you.
I like to go in clean clothes and shaven... you only get one chance to make a good first impression. Also, have an idea what you looking for, I say wheat cents, lead toy soldier, skeleton key... and have these items in my pocket to show them! Avoid the word "dig" or anything like that... people get the idea that you are going to dig trenches. Don't say "treasure" either.
My best weapon in my enthusiasm, people can tell that I am excited, and they get excited too.
I don't bring up splitting the loot, first of all, there isn't going to be anything THAT good anyway. If you find a ring, or personal item, show it to them and ask if they would like to have it. If it belonged to Granny, give it to them. I find that they usually laugh at me, they might want to see it, but have no interest in dirty pennies or anything else that I might find.
You might want to practice your schpeel once or twice. Don't get all excited, just go and talk to them. Look them in the eye, be honest and sincere. It's not all that bad. It gets easier after you do it a few times.
