Whats the best way to knock out a car door panel?

If the door panel has a crease like you describe, if you try to knock the dent out the paint will fracture from the panel and you will have to fill, prime, and repaint if you want to make a good job. But you could try ringing around local car breakers might be lucky and find a door undamaged the right colour, just a fraction of the cost for the repair, good luck hope this may be of some help.