I wouldn't put anything on the shell, even the "Conditioner" stuff you see at Petstores. Their shells need to breathe and when things are put on their shells (besides what a vet my prescribe) this can do more damage then good.

I soak my guys frequently which keeps their shell pretty clean and every once in awhile I'll take a soft bristle tooth brush and rub over the shell but only if there's cacked crusties.