This is one of those tips handed down through the generations father to son. At some point the verbal history lessons break down occassionally and things are missed like....well, like flashing headlights at oncoming vehicles to advise them a patrol car is just up the road looking for speeders... and, like, using dry ice to remove dings and dents.
This works on fuel tank and other dents that don't involve a "crease" in the metal. Purchase a piece of dry ice (frozen c02) at your local liquor store. It should be larger than your dent. Using thick gloves or a towel to hold the ice, apply it to the dented area. Hold the ice in place until frost forms at least an inch beyond the dent all the way around. Remove the ice and place the tank or metal part in the Sun if possible. If the trick works you'll be able to watch the dent vanish before your eyes. This also works to remove many parking lot dents in car doors.
See other quick fix tips under "One Liners" in the "Motorcycle Quickies" section.