The most appropriate place is around the neck of the animal.
It's amazing though how much the room it's in can change the sound.
My cowbell is mounted directly onto the hi hat rod, above the clutch.
I use it a lot, and usually in the same way as I use the hats, so it made sense to me to have it there.
E.g., my band plays Love Shack, which has a cowbell on 1 - 3 -1 -and- 3, and I just switch between hats and cowbell for those hits.
Yep. No rules.
I tried a bunch of different places mounted, and then decided to just plop it on the floor tom head.
As I get older, the positioning of things become permanently temporary.
The Tama clamp works well for me on the high hat rodI like that tama mount that Beatdat mentioned. I use a similar one that is made by Mapex. Hanging it off a cymbal stand gives you lots of flexibility in placement, and won't chew up your bass drum hoop. I move mine around on my kit, depending on what I have available to hang it off. Most of the time, I keep mine above my floor tom, but sometimes, between my hi hats and high tom.