We use bells to keep track of ours in the woods, as well. I like the tinkling of the bells (so do the dogs--when they hear it they get all excited!
) and there's nothing like the peace of mind you get from knowing where they are when they're out of sight.
We also use electronic beepers set to go off when the dog stops. The electronic beepers are much louder and we don't like the sound of them--but after one of our dogs had a seizure in the woods one day, we realized we needed some method of finding them if they go down.