Harry,,, the D.E.C. website used to have a "permission to hunt" form that you could download and print,,, not quite sure if it's still available. If it's not it's a simple enough document to draft.
As for what technique to use when approaching landowners,,,,,, I guess everyone has their own approach but presenting a good appearance, being respectful, honest and friendly are of course a given. I always make it a point to offer to share whatever game I harvest, some will take you up on the offer and some won't,, Also, once you receive permission a gift for the farmer and his family around the holidays always goes a long way to show how much you appreciate the privilege they have given you. I always make it a point to let the farmer know that I would never bring anyone else onto the property without permission and if granted permission I always ask the farmer what areas are ok to hunt on and what areas, if any, he wants me to stay away from. The other thing I always do is that if the farmer refuses to give me permission I just smile, politely thank him for his time and move on.
As you would expect most farmers are taken by surprise when I mention that I will be hunting using an air rifle. I always stress the fact that my "weapon" doesn't have any loud firearm type report and that for the most part my shooting will be fairly close range. Both of these facts always seem to be received well. I'm sure the fact that you have a law enforcement background would be received as a plus,,,, I know it always helps me.....
It's hit or miss,,, some farmers will gladly let you hunt,, some will just say no,,, but that's ok. Sometimes I'll check back with them the following year and they will say "ok".
I don't really concern myself with getting written permission here but that's simply because I work with all the county cops and the local DEC officers,,, however written permission is certainly the way to go if you can get it.
Jeff