from what I have found is the best way is to approch, introduce yourself, get to know the land owner, ask if you can camp for a night, some were he can keep an eye on you.. then the the relationship builds..
For example it took over a years work to get to talk to the owner of the land that the Suffolk Group use now.. and some long telephone conversations (1 hr+ some times), some emails to him with pictures of us and what we do.. that lead to a weekend for us he popped along to see what was happing that then lead to another weekend being booked.. then a few more weekends we are now 8 months down the road from the frist weekend, and by meeting with him after a weekend for a little chat, and some times arriving early and having a chat, also some of the guys from the group stoping to chat with his wife, and son have all helped...
As the relationship grows then what you can achive grows.. the last conversation has opened up the posiblity of being able to use most of his 1000 acres of wood land.. the frist trail of the extra woodland will be next year...
I must admit I was lucky to find such a landowner, but also I am lucky to be involved with a great bunch of guys that respect the woods and the landowner as well.