I've heard a lot about wearing the poppy 'over the heart.' Now, I'm not one of those people who get all riled up when people attribute emotions or whatever to a part of the anatomy separate from the brain but I will venture that this 'over the heart' explanation is false. I rather believe the placement of the poppy and the placement of the heart are coincidal. Let me explain:
1. Men's coats button in the front, usually with the left-hand side laying over the right (just a bit for single-breasted, more for DB). This was orginally to permit right-handed men (the majority) to draw their swords without catching them on the fringe of their clothes. This cut of coat/jacket therefore placed buttons on the right side and buttonholes on the left side of the jacket. Eventually lapels were cut into the common shape we see them in today's suits, with the top part folded back instead of up as in a high-collared tunic.
2. At some point after lapels were folded down, men decided to wear a flower in their buttonhole for style or whatever. As the majority of coats are single-breasted (except for us sailors) and cut for right-handed men, the flower was necessarily on through the left-lapel, near the top.
3. After the Great War, when poppies were worn, they naturally followed the place of all other flowers: in the lapel buttonhole. I imagine women would place them on the opposite side since that is where the buttonhole on women's coats is (presumably they don't have to draw a sword, or are all left-handed).
4. On headdress: I do not know if men wore poppies on their civvy hats as they would with military headdress. Perhaps not, because I do not recall hearing that civvy hats are as important as military headdress. Though various hats were once a key part of various suits as dress & undress caps are a key part of various uniforms. (Personally, I wear the poppy on my civvy hats if I happen to be wearing one during the beginning of Nov.)
Anyway, that's not the result of years of historical research, simply a different rational argument as to why the poppy is worn on the left. Since I believe this argument, I wish poppies came with wire stems so they could be placed properly in the buttonhole instead of pinned near it. I also get irritated (not a lot, I'm not obsessive!) when I see women having pinned a poppy on their left lapel, leaving a forlorn buttonhole on their right.
I usually shoot dirty looks at people starting 12 November who are still wearing them.