One thing I never go detecting without is a little note pad and pen....Lots of times when i'm detecting I see people out walking dogs and people just out for a walk, Its human nature for curiosity to get the better of most of them and invariably they will come across and ask if I've found anything??
I always make a point of showing them what I have found and during the conversation I ask them if they know any of the local land owners, most of them being local do, so I ask them for the farmers/landowners names and also what their names are. When I visit the farmer/landowner to ask permission I then add I have been talking to/know Mr whoever and I find this goes a long way in gaining the farmer/landowners trust
Not sure how relevant this is to any of you but i have gained most of my permissions in this way