Getting permission can be tricky. Alot depends on if the landowner has had a bad experience with relic hunters before or not. If one guy has screwed up and left holes or something, it ruins it for everyone after him. I joined the local historic association. That can give you some credibility on the more reluctant landowners. I also spend a lot of time talking to them. For example, I don't just get permission and leave. ALot of the landowners I talk with are older and just want to talk for a while. So I shoot the breeze with them sometimes for an hour or more and then go hunt. Always ask in person. Some websites recommend sending a letter asking permission. I have had zero success with that method. it is easier to turn you down in a letter or email than in person.Once you get permission, always leave the place better than you found it. ALways take your trash. After the hunt, show the landowner what you found. I sometimes offer some of it to them. Most of the time they don't take anything. I have given a riker mount with some of the items found on their property where I have found a lot of stuff. A couple of days after a hunt, I will mail a Thank you card with a little note expressing my appreciation for allowing me to hunt. If it is a guy that drinks beer, I will drop off a 12 pack. If it is an old lady, I have dropped off a tin of cookies. Some of the landowners relic hunt also, so I give them my old american diggers and north/south trader magazines after I have them. Sounds silly, I know but the landowner remembers you for that. Little stuff like that usually leaves me with a "come back anytime!" from the landowner.
Good luck!