Kreem Products - Kreem Fuel Tank Liner 1 Pint
you can buy a kit from "kreem" that has three stages. a metal etching fluid that'll roughen up the inside of the tank so it'll stick, a cleaner/degreaser, and the coating itself.
i've got it in my tank and it seems to work great.
only problem is, when your tank's all done and you want to go riding, you have to wait 24 hours for the kreem to set up. so you sit there and stare at it, willing it to dry faster, as it sits on your workbench mocking you.