The RSPB did a study not so long ago about this:

The RSPB: Environmental and process research: Cats and collars: reducing predation rates

I bought one of the sonic collars they recommended but it had a quick-release catch and she learnt how to leave it behind. It would still beep every seven seconds, so we could find it again, but every other day she would come back naked again so we went back to the bell.