When you go to talk to a landowner, show up on time, in clean neat clothes (don't look like a bum just off the freight train). Explain what you are seeking to do, assure them that any and all holes will be filled and ground brought back to original contours, any and all trash found will be picked up, fences and gates will be as found. If you notice anything out of place or an issue, let them know. Only bring friends if the owner agrees to it and make sure they are respectful of the property and rules the owner sets. Treat the property as your own, and how you want your property treated.
Can be helpful to offer to help out (like bucking hay bales from the field to the barn), etc. Above all, keep every word you give the owner. Longtime friendships with these people will form, and it will open open other doors with his friends when word of your trustworthyness gets around.
On the other hand, if the above is not followed, word will get around even faster and you will be out of luck.
**EDIT**---Bottom line is -approach the owner in the same way you would want to be approached if someone wanted access to your property.
Good luck in securing permission.