There is a tool, I can't remember what it's called, that welds a bit of metal to the center of the dent which you can then pull out. It isn't dangerous, doesn't hurt the tank (at least not more than it already is), and costs about five bucks. A well equipped autobody shop should have one. A little tapping around the edge of the dent really does help pull it out. (What was that thing called? It will drive me crazy until I find out!)