Besides the written permission, I also wait until the timing is right. In my case I generally wait until after we have had a rainstorm. That way I know the landowner/farmer will be home and not busy in the fields. I once stopped to talk to a landowner as he was working in the machine shed. Unknown to me at the time was just an hour before a shaft had broken on his combine.......big money item and he was not in a good mood. I backed off on asking at the time and waited a couple of weeks..all was well then.