[Scriptor]

That's a good point. However, is there much textual evidence that Frodo specifically built up resistance to the ring, and not just strength of character?

I guess it comes down to the following factors that might have an effect on ring-resistance:

    - strength of character/will
    - strength of body
    - proximity to Sauron

It's probably impossible to know exactly how much each factor contributed, but still good material for fun debates.