Creased stamps . Creases in stamps don't look nice. If the crease has damaged the surface design and the stamp is if low catalog value, it's probably best to just throw it away. However if the design is not damaged, it may be saved. As a rule, mint stamps are very difficult to get creases out of.
Used stamps are a lot easier.
There used to be gadget on the market where you placed the stamp inside and tightened the screws on each corner of the press. However, I haven't seen one advertised lately. Personally, I have found my Safe Signoscope electric watermark detector to be a very good device to use as a press. First you will need to soak the used stamp thoroughly, then grip the stamp across the crease with your thumb and forefinger and squeeze lightly. Very often this will be all that is needed to eliminate or reduce the crease.