Shortening and slightly radiusing the derimming punch has also worked very well for me.
Reducing the diam of that punch also reduces effort, but I find that if the diam is reduced too much, a variable amount of a slight fold or ring remains where the inside radius of the case rim was.
I prefer that the cases are not just derimmed, but ironed completely smooth. Not sure how much difference it makes, but I try to keep everything as consistent as possible.
That's just how I like it! Keep on keepin on!