Apply a suction cup to the lower half of the front panel, just above the home button.

Be sure the suction cup does not overlap with the home button, as this will prevent a seal from forming between the suction cup and front glass.

If your display is badly cracked, covering it with a layer of clear packing tape may allow the suction cup to adhere. Alternatively, very strong tape may be used instead of the suction cup. If all else fails, you can superglue the suction cup to the broken screen.