Hey DonJuan,
I have asked and have also been knocked back. Some too have given me entry and i can put it down to this.
1. Be as sincere as possible and don't go to the door with a detector in hand, presuming you are going to get in.
2. I let them know who i am and where i am from, showing them a drivers licence. It's only fair seeing you know them and where they live.
3. I usually have a bottle of wine or chocolates for the lady of the house to say "here is something for letting me on your property", even if nothing is found.
4. Let them know what you may suspect and its value, then offer a 50/50 split.
Have a short agreement form ready in your car.
5. If you are part of a club then let them know and show them a membership as it validates you from a spontaneous loony off the street.
6. Ask if not today would a future date be possible to come back.
These things have normally gotten me on properties and besides that you have to expect some knock backs as it's part of the game.
JustLucky