I don't know what everyone else will say to my advice, since this is our first puppy. Recently, I was reading the Monks of New Skete book and their advice on puppy mouthing is to stick your index finger into the puppy's throat which brings about the gag reflex. I started doing this the last two days(mine mouths once in a while nowadays, and mouths when I try to trim his nails etc.) He starts to nip then stops.
But do wait for others to let you know if this is a good strategy or not.
the other thing is perhaps to apply some bitter apple spray on your arms or legs or even pants. I use Bengay or Vicks cos bitter apple does or vinegar not faze our puppy.
The OFF does work though. Maybe just click and drop treat anyway. That way the puppy knows there is a treat coming once it backs off.