The extended firing pin and lighter hammer spring, polish the firing pin block plunger on it's sides, install the lighter firing pin block plunger spring and reduce the tension on the trigger bar spring. Don't reduce it too much or you might drop to half cock during slow trigger single action pulls.
This, along with all the requisite stoning and polishing will yield some good results. That's all I've done to my P-09 and the double action pull is very smooth and pretty light.