Perhaps you've heard of using boric acid dissolved in denatured
alcohol to keep firescale off of silver? Well, we use a ton of that
stuff in goldsmithing. The metal must also be very clean. Those are
your first steps. The next step is to make sure your flame is not
so high in oxygen, this will cause the metal to oxidize quickly,
burning off any flux you have applied. Then there's the flux,
although you can use regular brazing flux the do it, it works a
little better with self pickling flux, it's a flourescent green color
and has the consistency of water.
Hope this helps.