I had trouble getting some creases out of fabric even when using lots of steam until I was told about the vinegar trick. Now I keep a spray bottle of 50% white vinegar/50% water handy. It is amazing how easy it is to get rid of even the crispest creases. You can vary the percentage of vinegar to water and even use neat vinegar if a crease is proving very difficult to shift, but then you might have to wash the item to get rid of the smell.
I've not tried the fridge method someone here mentioned but it's worth trying as ironing isn't my favorite activity.