I haven't been doing this very long on a serious basis, only since I retired, but I've dabbled in it off and on for quite a few years.
The best way I've found to get permission on properties owned by older folks in my small town/rural area, hands down, is to talk to your older friends and/or relatives. Figure out who they know who might own a property you have your eye on, then get them to go make a quick visit with you for an introduction and to ask permission.
Most people here are willing to let you on their property if they know you, or know someone who knows you, it's tougher if you are a stranger to them. This may be the same elsewhere, if so, that's where I would start.
Then again, I've also gotten permission over the phone from a complete stranger, but that was for a vacant lot. It just depends on the owner and the property you want to search, I suppose.
I don't make a big production number out of asking, just who I am, that I do this as a hobby, and can I check around their place? I do let them know I won't be digging up the yard, just making small cuts and putting things back like I found them, and removing any trash I pull out.
Nobody has asked me for a share of finds yet, if they ever do that won't bother me since technically it's their stuff and they're nice enough to let me on their property.
Sometimes they do ask me to let them know if I find anything "interesting," then I just make a point of stopping back before I leave to show them what I found, and usually it's mostly junk that they didn't care about anyway, especially on some of these old farms I hunt.
