Here's what I do:
Find a tin can, plastic bottle, etc. that closely fits the tank end of the filler tube. Put it on the filler tube, and tightly duct tape it on there with multiple layers (also cap or plug any vent fittings). Then fill the tube all the way up with Evaporust or white vinegar ($1.25/gallon at the dollar store). Put the fuel cap on, stand it in the corner and let it sit a couple of days. Then drain it and thoroughly rinse it out. I don't care how crusty it used to be, it will look like new inside now. Install and enjoy.
