Ive done this with very fragile roman coins successfully. Easiest way was to set it on something like Playdoh, covered by foil, then put foil over it, another bit of playdoh and press down with a book over it. Coin comes out unharmed, deep imprint on the foil. You can also impress it with your fingers.
I was trying to make negatives of my nicer coins too, so I used oven baking clay and made a little die of sorts. Its nice becuase you see all coins as one colour all over.