Use a single suction cup to lift the front panel:

Press a suction cup onto the screen, just above the home button.

Be sure the cup is pressed securely onto the screen to get a tight seal.

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.