The cream kit is the way to go but I used to have to clean old tanks with a bunch of rust and crap in them. The cream I have never used but have seen it and have heard great things about it.
A half good fix is to take the tank off and put some rocks in it, then put a little kaerosene in. Take the whole thong and slosh it around and around and around. You will have to drain and refill it several times depending on the amount of rust. Use nickel sized rocks. The ones made of quartz in the street will do. You can shake, rock and jiggle the thing around making sure that the abrasive (thats the rocks) are rubbing against the walls of the tank all over.
Once you are finnished, you need to make sure that you put something over the surface of the inside of the tank to clean out the karoseen. I used the keroseen because it will help to break down any oil goop from having the gas just sit in the tank. Make sure that you put some gas treatment in the first tank to get any water out that may have stayed in the tank.