The method I used is crude, but it worked:
Remove the petcock and plug the hole.
Fill the tank with Purple Power Rust Remover.
Plug the gas cap hole with something to avoid damaging the rubber gasket on the gas cap; I recommend removing the gas gap altogether. Make sure it doesn't leak or acid will spill out on to your paint. If you want to be really careful you can mask the entire tank off with painter's tape.
Slosh it around, and let it sit for a while. If the inside is particularly horrible fill the tank with bbs and shake it around for a while before filling it up. Take the bbs out before filling it up.
Drain the Purple Power and rinse the tank with gasoline. The Purple Power leaves a "protective" residue that is soluble in gasoline. If you do not rinse the tank the residue will accumulate in your carbs. An inline fuel filter will NOT stop the residue.
This is what I did and my tank (and my friend's Honda) is/are both rust free after 1.5 years. I do, however, keep the bike in the garage and it never sees rain. It is fairly humid where I live. I do not make any effort to keep the tank full either.