I used a hot glue gun, a threaded t-cap, a long bolt, and a large diameter washer. I put the washer onto the bolt, threaded the bolt into the t-cap (looks like a very small top hat with a flat washer on one end with threads through the middle), applied hot glue to the tank, and stuck the t-cap up to it. Once it was cool (roughly 60 seconds), I pulled on the bolt (with the washer), and the dent came out.
If the dent is large, then start towards the outside first, and move in circles towards the center. You do not want to start at the center or you will warp the metal and it will never be flat.
I bought everything I needed at Ace hardware for $15 or so including the hot glue gun and glue.
Camelman