Snoogie, what kind do you have, do you know?
I have several that are floppy and drive me nuts. Mainly it's just that the soil I plant them in is way too nutritious for them.
However, I do have one fushia/magenta flowering yarrow that is shorter, and it NEVER flops. Blooms later and longer and I love it! This came from a GW trade in the mail, so I have no idea what it is. Looks fantastic not far from Phlox Nicky.
If you are coming to the fall plant trade I can have some for you.
You can cut yarrow back but I have found that most do not re-bloom in our climate. I'd love to find one that does!

GGG

I seem to recall that Achillea flops if it gets too much water. They really like it a bit drier, that's why I haven't even bothered with it since I have so many water issues.

I've got yarrow (Heidi) in the same bed as some lavender. No water, no fertilizer. They are standing up straight. Too much fertilizer will cause weak floppy growth.

I've got yarrow in a couple of my dry beds, and they're flopping over. They're pretty tall, tho. In contrast, I just purchased some shorter ones, and put them in my wet area, and they're erect. The white flowers on the taller floppy variety are so pretty, that I've been fairly successful at overlooking the flop. :)
Pam