Hi, first post to RIU, but I have been growing outdoors for just over 10 years and have formulated a great story that I have not been able to test.
Other than the obvious techniques that have been mentioned, here is what I do.
I carry a stack of these
http://www.geocaching.com/articles/Brochures/EN/EN_Geocaching_BROCHURE_online_color.pdf , a gps unit, and a backpack full of junk jut in case I have a surprise waiting on me when I get to the site. I have also joined a local geocaching association and carry their cards too. In addition, I have actually visited a few caches to obtain unique coins and such to stick in the backpack. I just mark a site nearby when I plant outdoors, put the geocaching symbol and a fake description and simply explain to the visitors what I am looking for. If you want, you can actually place a cache near it, and just not publish it to a site. I have gone as far as doing that with a forged log book to sign once the visitors decide if they are going to let me continue searching or make me leave the area. Anyways, this gives me a reason to be in the area. I mentioned earlier that I have not been able to test this story yet; however, I have had to explain geocaching to the local police while out looking for actual caches, and they never question me further.
Now, as part of my planning, I run the coordinates through the website to see if any are actually placed in the area. This way, strangers are less likely to stumble across my babies. They also have files you can open with google earth to see where the real caches are, and I tend to stay away from them as much as possible.
Other than this, the best advice I can give you, is use common sense, sound judgment, proper planning, and have a solid story to tell if you get caught in the area.