The original post in this thread discussed using motion sensors to monitor the yard for security reasons, and so all the various issues raised will apply to that type of use case.
Just recently, a conversation started in a different thread about a different type of use case, one more in the home automation area, such as having a light come on when you got to the first porch step. A much narrower detection zone than the security use case.
People in that thread have approached the problem by putting a plastic tube on the end of the sensor lens to narrow the detection field. That also addresses two of the common problems discussed in this thread, insects crawling across the lens and direct sunlight falling on the lens as clouds move. There should be much less of both of those when the tube is used. 
Anyway, I thought it was an interesting thread for those who were looking more for a specific “if someone crosses this exact point, I want to trigger a home automation event” rather then the more general “if there’s motion in the yard” security case.
And here is @eric182 's Photo from that thread showing the narrow detection sensor in place.
