“My house (primary residence) is on a lot without well defined borders to my neighbors. The grassy yard flows right into my neighbor property and there are some trees that act as a division but according to the plot plan my property runs well past them. Her lawn service cleans part of my lot (the part after the trees) and I worried she might think her yard is bigger than it is. How do I make sure after time she does not claim rights to the part of the yard she cleans?”
Easy: Give your neighbor permission.
The rights you’re concerned about your neighbor getting are known as a prescriptive easement. That is when you use the land of another person for long enough that you develop a protectable legal right to continue doing so. However, one of the necessary ingredients in a prescriptive easement is that the person must use the other’s land without permission. Granting permission defeats any later argument of prescription (assuming no rights have already been gained).
It’s best to do this sort of thing in writing, and if you’re really paranoid you could even have it recorded. It also wouldn’t hurt to check with a local attorney to make sure that you aren’t required to renew the permission every so often.