Zoe wears a "Safe Cat" collar that comes with a bell on it. You can also buy the little packs of bells in the cat section at Petco or Petsmart for other collars or harnesses. I like the fact that the collar will release if it ever gets hooked on something. We don't use it for walks or when out, then we use a harness, which also has a tag and bell attached. She's happy to wear her collar.
Occasionally I take it off if someone is sleeping, and she "goes stealth"
, but other than that it's always on.