Raspberry leaves do seem to work for me - there's a recent thread in the horse health section (I think) about them.
But, even with them, it seems like my mare is in heat constantly. Right now is particularly bad though, we don't have stallions where we board but that doesn't seem to matter. She's peeing and winking at anything that moves. She's also taken to pinning her ears at anything that even looks at her, which in some ways is comical because on the other hand, she's pretty much demanding that everyone look at her :) Poor girl-she's miserable and not fun to be around right now.