There are only two ways I can think of to remove the dent. One is to put a certain amount of water in the tank (off the bike, of course), and freeze it. I don't know what that "certain amount" is, though. If you put too much water in it, it will split seams and such, so proceed at your own risk.
A second method is to heat the area up with a blow dryer, then hit it quickly with a computer duster compressed air can which is turned upside down. Metal has a memory. When you run it through the quick heat/cool process, it tends to go back to its original shape. Depending on the size of the dent and if there are any creases in the metal will ultimately affect the outcome of how this might work. A crease will not come out this way. It may take several heat/cool cycles to get it all the way out.
There is a third way, but that involves buying a new tank, and I guess a fourth of taking it to a body shop to have them do it.