Every once in a while it is necessary to flatten a thin piece of metal, such as a piece of aluminum dash trim. The object itself might be bent, creased or marred with little dents and dings, and it's very hard to smooth such things out without an English wheel or great skill with rubber hammers.
A good way to get the job done is to place the piece between two flat pieces of wood (such as 3/4 inch plywood or straight 2x4). Position the piece carefully and then sandwich it between the wood blocks, clamping it solidly in as many places as you can fit your clamps, or weight it down with cinder blocks.
Leave the sandwiched piece alone for about 24 hours. When you take everything apart you will marvel at how nicely flat the piece has become. The wood surface is relatively soft when compared with the metal, so it won't hurt the finish at all.