Body work, in a car, or a bike, can be tricky. The metal is stretched in the dented area, and it cannot be "pushed back" into the exact same orientation it was orgininally. Steel tubing, especially thicker tubing, can be repaired fairly effectively with care. Aluminum is likely to become too brittle to be influenced by this procedure.
The 100-3C or 100-5C clamp from Park Tool use radiused jaws. Clean the jaw covers of any dirt, and then grease the covers. Remove the clamp from the stand. Begin by clamping gently on the middle of the dent. Clamp what should be normal pressure, with enough force to close the handle using only two fingers. Rotate the clamp around the tube. In some cases you can only rotate part way, so reverse rotation. Remove the clamp and check. Now, increase pressure slightly and repeat. This pushes the dent upward, but do not expect a "perfect" repair, it is best to stop after some progress.
NOTE: If the dent is in a high stress area, such as right behind the headtube, there is no good repair, even if the dent were removed completely. The metal will become work hardened in that area, and this may lead to cracking and failure.
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