Sending a letter sends a message that you are too busy to stop by. If I got one of those, it would go in the shredder immediately.

Cold calls to the home and hit the local cafes are the best. I have gotten permission by walking up to the door and speaking to the male by being polite. I have noticed here that in 90% of cases if I spoke to the woman, I would be turned down. On the other had, when I spoke to the man after a little conversation, I was approved. Build a report with them.Talk about their crops, how their looking pretty good/bad. Talk about growing up in the country. Talk about nuisances such as deer and turkey. Usually they will open up and feel comfortable then. Then ask to bowhunt their property to help alleviate those nuisances. Offer part of the meat. In local cafes around breakfast time, sit at the bar (if they have one) and start a conversation with the waitress about all the deer in the area. Talk to her about being a bow hunter and looking for a place to hunt. Either she'll know a farmer with deer problems, or one of the other patrons will have a problem with deer and approach you.

A classic example was years ago while muzzleloader hunting, we stopped at a local cafe for lunch and were talking about the morning's hunt. A man walked up to us and asked if we were deer hunting and he seemed quite upset. One of the guys said "Yes, we just stopped for lunch". He said, "I'll wait until you are done eating." We sat there eating lunch wondering what crawled up this guys sphincter. When we were done, he said get in your vehicles and follow me, I want to show you something. We followed him out of town and got to his farm. He told us to wait at the top of the hill and then drove down to one of his cattle feeders. He dumped several bags of corn in the feeder and drove back to us. By the time he got to where we were waiting, the deer had come in and were shoulder to shoulder at the feeder to where the cattle could not get close to the food. He looked at us and said "kill 'em all and if you run across any turkey out there, kill them too!"

Another approach I learned from the guy who got me into deer hunting is when approaching the farmer, if he looks like a down to earth person, is to walk up and say "Hi, my name is ........., got any deer you need killin'?" I about crapped my pants the first time he did that, but he always gets permission.

Another thing to check is the assessors site. You'll find land that is owned by people who don't live in the general area. Find where they live, find their phone number, and call them to ask. But always chit chat first.
I guess what I am trying to say is do your homework and read the landowners body language and make the appropriate approach.

Sorry for the lengthy response.

Good luck.